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