Sender or IP — which one counts?

Years ago, the first question we were asked by prospective customers was whether we sent from a “good” IP address. A good IP could decide whether a newsletter made money or not.

That is no longer the question worth asking, and it is worth understanding why.

A short technical explanation

Every device on a network has an IP address: your computer, your phone, your television. It is the device’s unique marker on the network, the equivalent of a telephone number it can be reached on. A server that sends email has one too.

In the old days, the IP address was the easiest way to recognise a server out in the world. The makers of spam filters worked that out quickly, so they gave the IP the most weight when judging the quality of a message. If the mail came from a good IP that had never sent spam, that counted in its favour, and the risk of the filter eating it dropped sharply.

Then the world ran out of IP addresses

IPv4, the standard behind addresses like 192.168.0.1, allows around 4.3 billion addresses in total. That makes the IP a poor way to identify a unique sender: there are not enough to go round. Senders end up clustered behind the same address, and a filter that weighs the IP heavily starts producing false positives.

The answer to the shortage is IPv6, which allows about 3.4 × 10³⁸ addresses — 340,282,366,920,938,463,463,374,607,431,768,211,456 of them, to be exact.

That solves the shortage, but it also means there are enough addresses for every single message to be sent from its own. The filters lose the ability to judge an address’s reputation at all, because a spammer can take a fresh one whenever they like.

What the filters weigh instead

Over the last several years, receiving systems have moved their weight onto the sender’s address and domain. newsletter@ubivox.com and ubivox.com matter considerably more than whichever IP the message came from — or more precisely, a combination of identifying factors that together form a unique identity for the sender.

It also means you can no longer get a free ride on a good IP address held up by other good senders. Instead you have to behave well yourself:

Do that, and the filters will treat your mail kindly — and you can worry a good deal less about the IP address you send from.