Shared or dedicated IP address?

By Mark Brownlow

If you use an email service provider (ESP) to send out your emails, do you insist on having a sending IP address for your exclusive use?

The theory...

Your reputation as a sender of legitimate marketing email helps determine whether ISPs and webmail services let your email get through to their users.

They associate a sender reputation with the source of the email. This means they assign a reputation (good or bad) not to "Mark Brownlow," but to the online source of the emails I send out.

You can broadly consider this "online source" as the formal, technical Internet address that identifies the location of the email's source. It is known as the sending IP address, which is used by my ESP for sending out my emails.

The sender reputation of that IP address (which is essentially my sender reputation) is determined by the practices of all those who send out email using that same sender address.

If only I send through that IP address, then my practices (and my practices alone) determine the reputation of that sending IP address. I am in control.

If I share the IP address with other senders using the same ESP, then their email practices also impact on the sender reputation of that address. So their email practices impact the reputation associated with my emails.

So if one of them is naughty, I suffer the consequences, too. ISPs and webmail services block the sender IP address of the culprit, which also happens to be my sender IP address.

That's the theory, and I've long searched for concrete references to explain whether it's true. Today, I found them courtesy of Melinda Krueger and Email Karma.

Melinda ropes in a few technical delivery experts to comment on the concept of dedicated and shared IP addresses from a deliverability point of view.

And Email Karma goes into detail on how email blocking systems relate to IP addresses.

Let me summarize my understanding and hope some technical folk can jump in with further comments or corrections if necessary...

The above theory is broadly correct. From a sender reputation and delivery perspective, it is better to have a dedicated IP address for your use alone. You are protected from being punished for the actions of others.

However, there are a few ifs and buts to consider (aren't there always?)...

1. It is not complete protection from sender reputation difficulties. Think of an IP address like a real physical address.

If you live alone in an apartment, then your behavior determines the reputation of that apartment. But if your neighbors are badly behaved, then the authorities (the ISPs and webmail folk) may decide simply to blacklist the whole apartment block.

Unfair on you, but it can happen.

In other words, people sending from an IP address close to yours can still cast a shadow on your reputation.

This is less likely if the ESP is proactive in ensuring its customers stick to best practices and itself keeps an eye on deliverability problems.

It can then step in and take corrective action before problems escalate to a level where "whole buildings and districts" are blacklisted, rather than just the "single apartments" that are actually causing trouble.

2. On the surface, being totally responsible for your sender reputation seems like a good thing. But this may not always be the case.

For example, if you've been slipshod about your email practices, then it may be that the good practices of those sharing your IP address have been keeping that address's reputation positive.

When you're on your own, your bad practices are not compensated for by others, and your reputation suffers accordingly.

This should not be seen as a suggestion to get away with bad practices by sharing a sender IP address (your ESP should hammer you if you do try this.) No, the point is that a dedicated IP address means you are more accountable for your actions.
Equally, if you don't send very much email, then it may be hard to actually build any kind of sender reputation with individual ISPs and webmail services. They need a decent amount of volume from you, sent regularly, to label you a "good sender."

In that case, a shared IP address lets you "combine" efforts with other senders to establish a reputation for your sending IP address (hopefully a good one.)
3. The scope of sender reputation is changing. As a response to phishing and other problems, the email world is trying to build authentication systems that allow email senders to be better identified.

There is growing emphasis on identifying senders not by their sending IP address, but by their domain name. As that concept spreads and is implemented, then your sender reputation becomes tied more to your domain rather than the IP address you send from.

At which point, the IP address question becomes less important. Then you really are in control of your reputation. And you can take it with you when you move from one IP address to another, or from one email marketing service to the next.

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