Server administrators who host mail accounts for multiple customers need to answer the question, “Should each customer have their own IP address or should they all share a single IP?” The answer might not be as easy as you think because both options have positives and negatives.

This question should be answered based on what is known as sender reputation. Sender reputation is how the major email providers rate an IP address based on its sending history. This is usually determined by the bounce rate, the number of spam trap hits, user complaints, and the volume of mail.

That means if one user on a shared IP address has a dirty address list, they will produce a lot of bounces, and in turn will hurt all others sending on that IP address. From this point you might say right away that all users should have their own IP address. But if you look again at how sender reputation is calculated one of the factors is volume.

One of the problems that most senders have with the sender reputation system is that your reputation starts off bad until you prove yourself based on the volume of email in relation to the amount of spam traps hits, complaints and unknown user attempts. This means that if you send less than 100k a month you may not have the volume to get a reputation high enough to send effectively.

There may not be one single answer to this question but knowing how things work allows you to find the best answer for your situation. No matter which method you choose the key is knowing what is happening and being able to adjust. With that being said you need to make sure your sending infrastructure allows you to see your customers bounce/failure rates so you can quickly react when problems occur.

Some hybrid solutions I have seen others use include…

Have mail going to the major ISP’s go through a shared IP and all others go through a dedicated IP.

Start all users on a dedicated IP and then when they prove themselves move them to the shared IP where they and other responsible senders can help each other improve upon the IP(s) reputation.

If you are experiencing email delivery problems or delay issues when sending to the larger ISP’s then most likely there is a reputation problem with your IP address.  The first thing any ISP will tell you when you ask how you can improve your reputation is to follow their best practices guide.  The problem is that every ISP has its own “Best Practices” guide and this can get overwhelming.  Below is a list of the top items that are common between the Major ISP’s (AOL, MSN and Hotmail.)

1.       Adhere to the CAN-SPAM Act – Senders should read the CAN-SPAM Act and ensure that their e-mail adheres to the requirements: http://www.ftc.gov/bcp/conline/pubs/buspubs/canspam.htm

2.       Clean your address list – After receiving a permanent non-delivery response with an error code between 500 and 599 you should remove the sender from your list.

3.       Secure your Mail Server – Having an open relay will get your IP address added to the major blacklists and cripple your sending.
 http://en.wikipedia.org/wiki/Open_mail_relay

4.       Branding – Use a consistent from header and include your company name in the Subject line, this will not only allow your recipients to quickly know who the message is from it also allows them to add your address to their personal white list.   Recipients who don’t know where the message came from are more inclined to hit the SPAM button, which is going to hurt your reputation with that ISP.

5.       Authentication – If you enable domain keys, DKIM and SPF this will cover you with major ISP’s that use authentication.

6.       Easy Unsubscribe method –  If the recipient does not know how to unsubscribe or does so and still gets another email message you can guarantee that they are going to hit the SPAM button.

7.       Use Opt-In Addresses only – Ensure that you are only sending mail to users who requested it. It is not advisable to purchase mailing lists or subscribe users by having an opt-in checkbox automatically checked on your website.