E-mail marketers and other senders are often flummoxed by an apparent disconnect between the Return Path Sender Score on their outbound IP addresses and their own deliverability metrics. A frustrated ESP sales team member posed me the following question this morning that illustrates this rather common complaint:

A client received zero complaints on a campaign to 30,000 recipients, but saw a 80% soft bounce rate at yahoo.com. Why would this happen? The IP reputation is solid! (98 score). Thoughts?

I’ll set aside for the moment other important aspects of the send that are not addressed in the question (like the rate of undeliverable addresses, etc.). Low complaint rates also have to be taken with a grain of salt. After all, no one complains about mail that’s already been delivered to the junk folder.

But for high volume IP addresses (like those used by ESPs), the Yahoo!s of the world rely less on the Sender Score – good, bad, or indifferent – when making automated decisions about whether to accept or bounce at the time of the server to server SMTP transaction. The reason they can (and often do) is because the big ISPs already have an enormous, statistically significant corpus of mail originating from the IP. They don’t need to rely strictly on Sender Score for reputational data. They’ve already got plenty in-house.

Sender Score looks at only a fraction of the overall volume from a given IP – traffic that goes to a relatively few seed accounts, and data from reporting partners. But the big ISPs of the world have a much bigger picture to look at in the case of high volume IPs. Plus, they know exactly how they calculate their own internal reputation score for the IP. Sender Score’s methods are proprietary, a bit of black box, even to the ISPs that use them.

That’s not to say that senders should ignore Sender Score; they shouldn’t. At the very least, the IP’s Sender Score is directionally relevant and provides a useful baseline to measure the effects of improvements made to sending practices, like list hygiene, permission methods, and other critical aspects of any mailing program.

Sender Score is very useful to the smaller recipient domains, who may not have seen much traffic from a given IP and don’t have a lot (or any) reputational data of their own to look at to help make automated delivery decisions.

While Return Path states that IPs with scores above 75 should enjoy acceptable delivery over a broad cross-section of the Intarwebs, my own experience has been that senders will notice significant deliverability problems with scores at or below 80. When a sender sees their IP’s score headed in that direction, they should take it as a sign that something is seriously wrong with their current sending habits.

Marketers new to the e-mail space often assume that Sender Score is the only important reputational metric to consider. It’s easy to understand how one might reach that conclusion; Return Path has an enormous footprint, and will show the score they’ve assigned to any particular IP, free for the asking. It’s an incredibly useful tool to have in the toolbox, but it’s not the only one.