SMTP and Email Delivery 101: When Is An Email Actually Considered Delivered?

smtp email delivery

Have you ever wondered what it actually means for an email to be considered delivered?

For example:

Is an email delivered when it reaches the inbox?

Or, is an email delivered when it is opened and read?

Today you’re going to learn about SMTP and email delivery — exactly when an email is actually considered delivered.

SMTP Is Just One Part of the Email Delivery Puzzle

Sending email via SMTP, or Simple Mail Transfer Protocol is the email standard and used universally to get an email to a mail server, but what happens after the message is accepted by the receiving server involves additional factors that SMTP doesn’t necessarily provide. This includes things like deliverability, sender reputation, authentication, tracking, and more.

Simply put, even if you are using the most robust SMTP server, your email may not necessarily be delivered.

This brings us to a big question when thinking about SMTP and email delivery:

What does it even mean for an email to be considered “delivered” in the first place?

To answer this question, let’s start by discussing what email delivery means.

What Is Email Delivery? (Meaning of Delivery vs Email Deliverability)

It’s easy to confuse “email delivery” with “email deliverability.” So, let’s start with some simple definitions to help explain the differences between the two:

Email Delivery: When a message is sent to a receiving mail server over SMTP, the mail server must choose whether or not it would like to accept the message. This happens before the message is placed in the inbox or spam folder. Therefore, “delivery” refers to whether or not the receiving email server accepted the message.

So then, what is email deliverability?

Email Deliverability: This term refers to what happens after the message is accepted by the receiving email server. For example, whether it’s placed in the inbox, spam folder, or somewhere else. Deliverability is more related to inbox placement. If you’re having trouble reaching the inbox, then here are some best practices to help you improve email deliverability.

Why is delivery important?

Email delivery is extremely important, especially when sending business critical email like password resets, status notifications, and time sensitive alerts. This is because if the emails land in the spam folder, then they aren’t useful to your customers or you.

Now that you have a basic understanding of what delivery means, let’s explore when an email is actually considered ‘delivered.’

When Is an Email Delivered? (3 Common Answers)

If you ask someone when an email is considered ‘delivered,’ then don’t be surprised if you receive a number of different responses. Here are three of the most common answers to this question.

An email is considered delivered when:

  • 1) The email is accepted by the receiving mail server: In this case, the message leaves the sender’s mail client and is successfully received by the mail server. However, there’s still more work to be done on behalf of the receiving mail server to determine how to properly handle the message. This scenario is equivalent to sending a package to a mailroom at an office building. Even if someone signs off on the package, it doesn’t mean that the intended recipient received the package.
  • 2) The message is placed in the appropriate mailbox of the receiving mail server: In this situation, the message was not only received by the mail server, but the message was also placed in the appropriate mailbox. Going back to our mailroom example, this is similar to sending a package to a mailroom, having someone sign off on the package, and then delivering the package to the intended recipient. However, this case does not account for whether or not the recipient opened the message.
  • 3) The recipient opens and reads the message: Finally, this case takes the answer for ‘when is an email considered delivered?’ to the extreme by saying that the recipient actually opened and read the message.

Since the meaning of “delivered” varies depending upon the situation, context, and email service provider (ESP), all three answers are correct. However, from a technical perspective, there is only one correct answer.

The Technically Correct Answer for When an Email Is Delivered:

Technically speaking, a message is considered delivered when your mail server receives a “250 OK” SMTP response at the end of an SMTP transmission.

A server response “250 OK” indicates that the SMTP communication was successful and it’s issued in response to every accepted command (likely 4 to 6 times per message). For more information about SMTP Response Codes, read our latest blog post: 21 SMTP Response Codes That You Need To Know.

As a sender, you will almost never notice the “250 OK” response, because this code happens in the background after an SMTP transmission. Therefore, it’s safe to consider a message delivered when it arrives in the target mailbox.

A good email delivery company or SMTP relay service, will provide you with failed and delivered message reporting, so you can easily check your email deliverability, without having to dig into your logs for a “250 OK” response code.

smtp email delivery

That wraps up our blog post on SMTP and email delivery.