Email Performance Red Flags: Spam Complaints

spam complaints

Welcome to a new SocketLabs series: Email Performance Red Flags! 

Now, we’re all familiar with “red flags,” where there is a behavior or sign something isn’t quite right. Some of us are better at spotting them than others, but when it comes to email, getting skilled at identifying red flags is an important key to improving your email marketing success. 

We’re going to collaborate with the brightest and most knowledgeable minds in the email community to help you hone your red flag detection skills. We’ll bring expert opinions, anecdotes, and guidance on multiple topics over the next few months, including unsubscribes, open and click rates, blocklistings, and much, much more. 

Let’s start with one of the most important indicators to email performance. They’re incredibly damaging to deliverability and make reaching your email goals a whole lot harder, so this is a great place for us to start practicing red flag detection. 


Today, we’ll tackle one of the most major of major email red flags: spam complaint rates. One of the challenges about complaints is that they’re not necessarily easy to detect or follow. Plus, you don’t only need to worry about high rates – a suspiciously low spam complaint rate could also indicate an issue. 

Let’s untangle this messy web of circumstances so you’ll be able to spot this flag flapping in the breeze. 

But First, Our Experts 

Soon you’ll see quotes and opinions from three well-regarded email experts in our community: Alison Gootee, Marcus Biel, and Will Boyd.

Alison Gootee is the compliance and deliverability enablement principal II at Braze, where she’s spent five years mastering all things email. Before Braze, she was in compliance at Campaign Monitor (Emma) for more than two years, and before that, she was advising at MailChimp. This year marks 10 years in email for Alison! Fun fact: she’s also the email meme queen of LinkedIn. There are too many good ones to share, so just go take a look for yourself. You will not be disappointed.  

Marcus Biel is the founder at Maildroppa, a GDPR-compliant email-marketing-app he’s been building with German-engineered precision for the past 3+ years. When he’s not building tools to enable bootstrappers and single-founders to grow their businesses, he’s changing diapers, drinking coffee, and keeping his wife happy…not necessarily in that order.  

Will Boyd is currently the director of deliverability services at Simon Data. Previously he spent more than four years doing deliverability and email consulting at SendGrid, and more than five years at Campaign Monitor (now Emma), making him more than a decade-deep into email. And it shows! Here’s an example from our Email in 2025 series.  

These folks clearly know the ropes, so we’ll be peppering in their takes throughout. You should take their notes! 

Red Flag 1: High Spam Complaint Rate 

First up are high spam complaint rates because this is a pretty clear indicator something is wrong. 

What Can Be Considered “High?” 

What you consider a “high” spam complaint can be very subjective, because something tolerable to you could be completely unacceptable to someone else. 

Generally speaking, you could go by what Google considers “high” and avoid ever going any higher than 0.1%. If you surpass a 0.3% rate, you could find your mail blocked by Google. 

Here’s the tricky part though: a lot of senders have been scratching their heads over fluctuations in their spam rates within Google Postmaster tools. So, we asked our experts to weigh in on what they consider to be “high”: 

“The other day, I saw 14% reported in Postmaster Tools,” said Alison.  

While that feels intensely high and likely (hopefully?) was an outlier for that sender, spikes above 0.3% don’t seem all that uncommon for senders following best practices…particularly on days when they haven’t sent anything.

Will’s perspective sheds some light on why this might be happening: “A handful of complaints for 5,000 messages can seem astronomical in terms of rate, but the overall impact can be widely varied based on the mix of positive signals the inbox provider sees.” 

Ultimately, when it comes to what you should consider a “high” spam complaint rate, be sure to keep in mind there are many sets of rules you need to follow: the infamous rules recently published by Yahoo and Google, the less newsworthy but just as relevant rules enforced by all of the other mailbox providers around the world (including Hotmail, the second largest provider in the world, with more than 400 million active users). Not to mention, the rules you’ve agreed to within your ESP’s terms of service. 

So, much like everything in email, it depends! 

My advice? Even if some of the most noteworthy mailbox providers are saying to keep it below 0.1%, do not use this as your target if you want to reach the inbox consistently! Aim to keep your spam rates consistently below 0.02 – 0.03%, and no matter what your rate is, always look into any campaign that generates a higher rate (or count) of complaints than normal. Identify what you believe might be causing it and fix it before it impacts your performance. After all, it’s a whole lot easier to stay in the inbox than it is to dig yourself out of the spam folder.

What Causes High Spam Rates? 

There are several reasons you might confront high spam complaint rates and while some are in your control, unfortunately, others simply aren’t. You should make sure to keep close control of the things you’re able to influence to counter the things left up to the whims of your recipients. 

As Marcus shared, “Many different factors cause high spam rates. Some of the main ones are poorly targeted emails that lack personalization, misleading subject lines, subpar list quality, or simply bombarding your audience with too many emails.” 

Will has found that “The vast majority of spam complaints are generated by mail that recipients don’t feel like they’ve signed up to receive. When someone sees a message from a sender in their inbox that they don’t recognize as a brand they’ve opted in to get email from, they are exceedingly more likely to report that message as spam.” 

You might not be able to change a recipient’s bad mood on any given day, and you may never know for sure what could trigger them to mark your email as spam, but you can make sure you’re following email best practices to increase the likelihood that the email doesn’t feel like spam. 

List Quality 

Your list quality has a direct impact on your spam complaint rate because your collection practices set the stage for your recipient behavior. Some problematic choices include not getting permission before emailing them, not setting expectations about what your email will provide, and in some cases, not sending to your recipients often enough for them to remember who you are and why they want to hear from you! 

Validating your addresses when they’re given to you can help to ensure you collect a real, working email address. There are a couple different methods for this. First, you can pay to run your lists through a service who will cross-check the addresses against verifiable sources. This is a good option for organizations who don’t have a person dedicated to analyzing and actioning email performance metrics or those who send infrequently and want to ensure they have the most up-to-date list possible. You might also want to prioritize a list validation service if you’ve recently acquired a new company and are unsure of the quality of their mailing list(s). We’ve…seen some things. 😳

The second option is usually more feasible in today’s economy: using confirmed or double opt-in standards. Let’s say you collected an address via a form on your site. Instead of immediately adding them to a common distribution list, or “single opt-in” them to your list, you can send an email asking them to take an action to confirm they do want to receive email from you. 

There are a couple of benefits to using a confirmed opt-in (COI) process: 

  • You’ll find out if an email address actually receives mail or if it’s invalid (i.e. a hard bounce).

      • If your email makes it to into the inbox, you’ll be able to see are if the recipient opens the mail, confirming they are engaged with their inbox. That’s a good sign. 

        • You’ll also know if they followed your call-to-action to confirm their email address, meaning they actually want to receive mail from you. Winning!

          If they don’t take that final step to confirm their email address — even if they do open the message — it’s possible that while your email did go to a real human, it didn’t go to the right human. People entering the wrong address into subscription forms is more common than any marketer wants to admit. Alternatively, they may just not be that into you after all. Which is ok! Because either of these types of folks could turn into spam complaints if you had simply added them to your list before confirming they want to hear from you. 

          We’ll tie this section up with sage advice from Will: “Engineer practices and processes that put the recipient and their wishes at the center of your strategy. Collecting addresses and preventing spam complaints is a function of properly and accurately setting the recipient’s expectations around who they are signing up to hear from and what they should expect to receive. While 90% of spam complaint prevention is expectation setting with the recipient, the other half is continuing to exceed those expectations.”

          List Maintenance 

          Speaking of expectations, you need to keep a clean list. No two ways about it. When you get unsubscribe requests and recipient-level spam complaint reports, you need to suppress them immediately. If you don’t, you greatly increase the risk of your next email to that individual being marked as spam.  

          You’ll also want to continuously clean your lists. Remove unengaged parts of your email audience, be diligent about how you handle hard bounces, and make sure you’re letting your audience be your guide. Remember, just because you don’t think your email is “spam” doesn’t mean your recipients agree with your assessment. 

          Marcus shared an example of how important it is to maintain clean lists, particularly during the migration process from one ESP to another.

          “Maildroppa only offers double opt-in, so the email lists built with us are generally clean. Problems may arise when customers switch to us from other providers, sometimes bringing a low-quality list with them. Then, when they send their first newsletter, we see an unusually high spam rate. The most common excuse is that the email list has gone “cold” and needs to be “warmed up” again. But, even if this is the case, a cold email list is a problem on its own.”

          Disappointing (or Enraging!) Your Audience 

          It should come as no surprise that people don’t like it when reality doesn’t match up with their expectations. But one of the most influential causes of spikes in spam rates is using email marketing tactics that confuse, disappoint, or enrage people. This behavior can be well-intentioned and not at all nefarious, but people can be fickle. If you do something they don’t expect or aren’t happy about, you could find yourself in a sea of spam complaints…and then, in the spam folder.

          Here are a few ways you may be asking for a spam complaint without knowing it: 

          • Sending with an irregular cadence or contacting recipients after a long silence could mean they forget about you or stop caring about what you have to say. Send it right, and send it regularly.

          • Using content not aligned with the type of email you promised. For instance, advertising an educational newsletter and then sending daily deals instead of resources they can learn from. 

          • Using misleading, vague, or aggressive subject lines. This can backfire if the message inside doesn’t line up with what they expected. Especially avoid using “Re:” or “FWD:” in subject lines because they are widely understood as “tricks” and can very quickly result in an avalanche of complaints. Talk about rage!

          • Not making it easy to get off the list. Unsubscribing should be easier than complaining, so if you make it too difficult to complete the process, you can count on them complaining instead. I’m adding a quick plug here for the one-click unsubscribe required by Google and Yahoo starting June 1st, because it makes unsubscribing easy for recipients. In fact, during our recent webinar with Yahoo and Google, Clea Moore (principal product manager for Yahoo) mentioned that senders are seeing a 30% decrease in spam vote rates, simply by adding this mechanism. So, like, stop worrying about healthy list churn and do it. Now. We’ll dig deeeep into unsubscribes in our next edition of this email performance red flags series, so stay tuned.

          All of these activities can have an immediate impact on your performance, so sign up for your own email program to test the experience. Does everything work as expected? Does the recipient experience make sense, and align with where you signed up? Are you as happy receiving that mail as you were sending it?

          According to Will, “When recipients are reporting your messages as spam early in their receiving journey, that tells you that something is misaligned with what you are sending and recipient expectations. In these cases, even a mildly elevated complaint rate of .015% can be an early indicator that recipients are also very likely to start out their journey being unengaged or producing other behavior that mailbox providers might measure as a negative signal.” 

          Alison has one of the best examples of a sender not reading the room and finding their tactics more harmful than helpful: 

          “Once upon a time, I worked for an ESP that had Mario Batali as a customer. Some bad behavior of his came to light, and when he addressed it via his newsletter, he included a recipe for cinnamon rolls. It was so tone deaf, it even made headlines as people debated whether that was appropriate (and whether the recipe was any good). That campaign got a TON of spam complaints, and I got to handle the ensuing Compliance case.” 

          How Does a High Spam Rate Affect Performance? 

          The consequences of a high spam complaint rate are varied because there are 100’s of factors potentially affecting your deliverability. You’ll (usually) see their impact from different angles.

          For starters, don’t be surprised if your ESP sends you a warning as your spam rate (or count) increases. Most ESPs have policies around the quality of mail they’ll allow you to send via their service, so if you intentionally (or unintentionally but refuse to improve) send mail frequently described as “spam,” you could find yourself without a way to even send the mail. 

          “One of our primary goals at Maildroppa is to achieve the highest possible delivery rates. This means that we check every customer who registers with us. […] If we come across a spam complaint rate that’s above 0.01%, we take a closer look at the email in question. If there are any doubts about the email, then we’ll have a discussion with the customer about it. We try to be extra safe because, we’d rather work with customers who are really “clean” to protect the reputation of both our customers and our platform.” 

          What Marcus is saying here, is if you don’t shape up, you could find your ESP suspending or even terminating your account from their platform. 

          Additionally, you’ll face performance issues at receiving mailbox providers who are only worried about their customers’ experience with their inboxes. Regardless of how valuable you think your mail may be, they’re not going to risk having their customers leave for a different mailbox provider because they’re perceived to be allowing in too much spam. 

          What happens then? 

          “The likelihood of your emails reaching the intended inboxes is greatly reduced. If the emails aren’t opened, all your effort in creating them is for nothing. This affects your conversion rates and wastes valuable marketing resources,” said Marcus. 

          “A high spam rate can also damage your brand’s reputation. If subscribers start associating your emails with spam, they’re going to lose trust and loyalty in your company. This isn’t just a problem for the sender, it also damages the reputation of the platform that’s used to send the emails. That’s why at Maildroppa we’re so selective when it comes to our customers. We actively monitor spam levels to make sure that our platform stays reliable and effective.” 

          Right on, Marcus. Totally agree. 

          How Do You Catch A High Spam Rate? 

          You should always, always be looking at your email performance data. Most ESPs report on your spam complaint rate using feedback loop (FBL) data provided by Yahoo, Hotmail, and many other mailbox providers. This can be helpful in seeing recipient-level spam complaint data to determine which emails are generating the most complaints.

          Though Google provides aggregate data instead of a traditional FBL, if you know you have lots of Gmail domains on your list, you can refer to their FBL to see if complaints are piling up there. You’ll also want to monitor your spam rate through Google Postmaster Tools.

          Since not all mailbox providers offer recipient-level FBL data, our Complaints report surfaces insights at the destination-level, by From domain, so you can catch provider-level issues you might miss by looking only at your overall spam %. Imagine, if your list is 60% Gmail, your overall spam % is going to be look artificially low, even if spam rates are very high with the other top providers you’re sending to. One of my favorite parts about this report is that it visualizes the top mailbox providers you’re receiving spam complaints from so you can see how those are trending over time and deal with them, fast. Take a look…

          spam complaints report

          Beyond spam complaints, there are other common metrics that could signal a complaint issue.  

          “A decrease in positive engagement like clicks can precede an increase in spam complaints, but I like to keep an eye on all metrics, all the time, and consider their relationship to each other in the context of the sender’s traffic,” said Alison. 

          “For example, a rising spam rate in tandem with a growing open rate might mean that inboxing has increased, which is great! Of course, if the complaints are consistent or even grow day over day, you’ll want to reconsider your volume and engagement criteria lest you land back in the dreaded spam folder!” 

          Unsubscribes are one of my personal favorites, since they’re another form of negative engagement from email recipients suggesting spam complaints might also be high — only unsubs don’t negatively impact your deliverability! We’ll be talking all about unsubscribes in the near future, but generally speaking, if you see them go up — particularly above 1% — try to find out why. There may be a spam complaint issue on your horizon.

          From the ESP compliance perspective, Marcus shared “the highest spam complaint rate that we’ve ever measured was 2.63% with a total bounce rate of 6.58%. We can’t say what the open and click rates were because the customer had deactivated open and click tracking. This in itself can also be a red flag for spam.” 

          Ultimately, looking at your overall performance can paint a complete picture. We encourage our senders to use their StreamScore as a starting point for investigations since it incorporates 26+ data points including your delivery and engagement stats, plus 3rd-party data points like postmaster and spam trap data feeds, bounce responses, seed testing, and more. As you can see in the example below, you would very quickly see your Engagement score and overall Streamscore drop quite a bit if you sent to a new (ahem, not so great) list and your complaint rate spiked. This would be your indicator to jump into your sending statistics to start diagnosing the problem.

          A view of the guided insights available through SocketLabs Streamscore.

          Red Flag 2: Low Spam Complaint Rate 

          But wait, isn’t a low spam complaint rate good? Unfortunately, not always. If you’re seeing a 0% spam rate, it might be time to apply the adage: If it seems too good to be true, it probably is…a red flag. 

          Alison reminded us, “Counterintuitively, those 0.00% spam complaint rates can indicate trouble. If mail is going to the spam folder, then people can’t report it as spam, and those zeroes don’t look quite so good!” 

          To be sure, you can have a low spam rate and it’s because you’re doing a great job at email! But you always want to make sure you’re reviewing performance from all angles to ensure you’re not lulled into false confidence. 

          “Low spam complaints alone aren’t a clear measure of success. You need to focus on metrics that directly reflect engagement and conversion rates, such as open and click-through rates and your conversion rate,” shared Marcus.  

          “If those values are low or dropping, it’s an indication that the emails aren’t reaching your target group properly. This could be due to issues like poor quality content, inadequate targeting, or bad email frequency. These problems can prevent your campaign from achieving its goals, even if you only have a few spam complaints.” 

          Signs Your Low Spam Rate is NOT a Problem  

          Let’s start with the most positive take possible: Your audience might love your email! You might have a great fit between your content and the people you’re sending it to.  

          If this is the case, look for: 

          • Positive engagement like opens, clicks, and replies

          • Increases in conversions, web traffic, and revenue 

          We love our StreamScore in these situations

          Though…if you’re not seeing these green flags waving in the fresh breeze…the next section is for you. 

          Signs Your Low Spam Rate is a Problem 

          How do you determine the low rate is a problem? As always, look to your data. Low open and click rates paired with low spam rates is not usually a good sign.

          Downstream metrics also can be directly impacted by spam problems. If you have a 0.00% spam complaint rate but you’ve also noticed low conversions, or a lack of web traffic spikes around deployment times, that zero you’re smilin’ at could actually be a waving red flag.  

          Will has first-hand experience with this: “More than once I’ve worked with senders who identified a decline in spam complaint rates with no real changes made on the sender’s side as an indicator that things were improving and good times were on the way. Unfortunately for them, the reality was that other measures, such as declining open and click rates, were all pointing to a marked decrease in actual human interaction that was almost certainly due to increasing percentages of their mail going to the spam folder.” 

          The Impact of a Problematic Low Spam Complaint Rate 

          While it might hurt to realize your low spam rate is a symptom of a problem rather than a sign of good email health, your real problem becomes the business impact of your emails not reaching your recipients. I’d love to say more here, but it’s really pretty simple: no opens and clicks means no conversions or revenue from email.

          Managing Spam Complaint Rates

          “The best place to start is by monitoring your email campaign reports so that you can track the rates of spam complaints. You should also react quickly because spam complaints are serious and neglecting them can damage your email deliverability and reputation,” warns Marcus. 

          Use tried-and-true best practices. Maintaining quality list hygiene, solid opt-in practices to ensure consent, and segmenting your lists for better personalization opportunities. 

          “Get permission, always,” said Alison. “You can’t pull a lever and make complaints go away, but you can make an email program so strong that no one on your list wants to report your mail as being junk!” 

          To Alison’s very good point…remember you’re sending to humans! People want to be understood, so make sure you’re using all available data to understand their needs and interests. Send content that you genuinely think they will find interesting, helpful, and worthy of their attention. 

          “Continuously try to improve your strategies based on feedback,” included Marcus. “If you need to, get professional advice to help reduce spam complaints and protect your reputation.” 

          Also stick to what you promised! No one likes being bait-and-switched. And people also like routine, so make sure to send with regularity. Remember to make it easy to unsubscribe so it isn’t easier to complain, and like we said, don’t use any weird trickery in your messaging. Nobody likes that.

          “Truly preventing damage from spam complaints isn’t simply about limiting the number of complaints mail generates. Rather, the important ratio to drive is active, greater positive engagement compared to spam complaints and lack of engagement for messages that actually reach humans,” reminded Will. “Engineer practices and processes that put the recipient and their wishes at the center of your strategy.” 

          Investigating an Issue 

          It’s probably pretty clear by now that preventing spam complaints is the best way to go. But, what if you’re already facing an issue with spam complaints?

          Well, get to work, my friend! Spam complaints are incredibly damaging to deliverability because they are a signal — directly from the recipient — saying “I don’t like this. Make it stop.” And the mailbox providers are more than happy to service that request to keep their users happy. So, stop what you’re doing and spend time reviewing what happened in the few days leading up to the spike in complaints. 

          Here’s a shortlist of questions to guide your investigation: 

          • How did you discover your issue? Low open rates suggesting spam folder placement, a spike in your spam % in Google postmaster tools, a friendly slap on the wrist from your ESP. The answer to this question will help you determine if it’s a false positive or a real issue. Providers have made failure responses increasingly specific, so If you’re seeing deferrals or bounces, use that information to help you isolate the source of the issue. 

          • How widespread is it? Did you notice a problem with one particular mailbox provider or several? Is it affecting one recipient or many? The path to resolution will look different if you’re facing an issue at Gmail vs. Hotmail vs. a B2B destination in Europe. 

          • When did it start? Separate question from how you discovered it. Go back a few days before you noted the change in spam rates to see if any of your metrics look amiss. Was there a specific campaign or segment or content type that had a higher-than-normal spam complaint or unsubscribe rate? 

          • How do addresses end up on your list? Data quality is a main driver for deliverability issues, so be sure all of your address collection sources, lead magnets, and expectation-setting is buttoned up. And test the program yourself! Sometimes what marketers think is happening with their sign-up process or automation series is…not what’s actually happening at all. 

          • Did you change anything recently? This is a big one. Targeting, frequency, content, list cleaning schedule, list collection sources or lead magnets — anything. Sometimes it’s something you didn’t change but another department did, such as removing DNS records during a platform release or maintenance. Cast your net and see what comes in. 

          Resolving Your Spam Complaint Issue 

          Will shared an experience where “recently, I began working with a long-established brand that, in spite of low complaint rates, had a history of deliverability challenges such as reputation-based blocking at Gmail and severe spam filtering at Yahoo and Hotmail. By identifying addresses by the different manners in which they were originally collected (website vs call center, etc.), my client was able to identify those collection points with exceedingly high complaint rates and create different journeys and messaging for those recipients.” 

          He added that “smarter targeting limited their exposure to spam complaints by limiting the number of messages and even boosted positive engagement early in the subscriber journey. This segmentation and welcome strategy was a critical addition to more aggressive sunsetting practices that helped them speed up the time to repair across the variety of mailbox providers. Within 3 months, the client went from chronic inboxing woes to open and click rate improvements clearly pointing to more and more mail reaching recipients.” 

          Alison shared a success story of her own: “A daily deals sender I was working with was seeing consistently high spam complaint rates. They just couldn’t figure out why it was happening. I poked around their user histories a bit, and noted that a lot of the complainers were coming in not from their Daily Deals signup, but from the newsletter footer on their website. The sender had never really connected the dots before, but I showed them how all of those subscribers not only came from the same source, but had similar entries for their First & Last Name fields; just random alpha-numeric strings. You probably know where this is headed! The sender was dealing with list bombing and hadn’t realized it. They were so happy with all the new signups, they didn’t stop to think that their growing subscriber base was increasing a bit too fast without the expected return on investment (ROI) lift. A captcha and COI later, and things were back to normal! This sender learned the hard way that a bigger list is not always better.” 

          Ultimately, the best thing to do when you notice an issue with spam complaints is to breathe, run through your checklist to collect some facts, and then determine what your next steps should be — if there are any! It’s always possible that high spam rate (or low spam rate) you saw in your ESP stats or in Google Postmaster tools is a false positive; these are particularly common when using deliverability monitoring tools since they typically make use of third party data points like seed and panel data, spam trap feeds and blocklist monitors in place of your first-party email data (e.g. deliveries, opens, clicks, unsubs, complaints). Relying upon reality-based metrics such as your ESP statistics and other sender health metrics like Streamscore is going to save you a lot of time and frustration. 

          And if it turns out that spam complaint issue is real, at least now you’ve got a whole bunch of tips to help you through it. 💌

          The More You Know… 

          You’re now a master at spotting a red flag: spam complaints. How does it feel? With this knowledge, you can spring into action at the earliest moment to identify emerging issues, resolve any problems, and repair potential damage in the blink of an eye.  

          We won’t stop here. Next time, we’ll help you spot another nefarious email red flag… unsubscribes. But wait, don’t jump to conclusions! Unsubscribes can be a good sign, too. So, we’ll walk you through what makes them a sign of danger, how to figure out if you should be concerned, and what to do next in each circumstance. 

          If you need some more help with an email performance issue — spam complaint or otherwise — let us know and we’ll set up a free consultation. We can help you make sense of what you’re seeing and plan your next steps.

          Until next time! 

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