This is another one of those entries that I write for people who are never going to read it, but I don't care; I just feel like pointing out the relatively obvious.

Suppose that you are someone who runs a mailing list service. Like everyone else who offers such a service, spammers will attempt to (ab)use it. Thus, one of the important things that you need to do is detect signs that you have a spammer's mailing list, and these days you certainly can't count on abuse complaints to tell you this.

As I've mentioned before, SMTP time rejections can be an important signal. The corollary of this is that the kind of SMTP rejection matters, and in particular you should really pay attention to MAIL FROM and DATA rejections and consider them a significant warning sign. This is because there are many fewer reasons for rejecting at those stages than for rejecting at RCPT TO time so if your mail is rejected then, well, there's any number of explanations besides 'it's spam'; the user's account could have expired, for example.

(And, let us admit, a disturbingly large number of mail systems have temporary glitches that cause equally temporary RCPT TO failures. This is why real mailing list management software pretty much never automatically removes addresses on a single RCPT TO failure.)

Since they don't have these relatively innocent explanations, mail rejections at MAIL FROM or especially from DATA are often signs of something serious going on. In particular a permanent failure at DATA time almost invariably means that the recipient's system really dislikes the message for some reason; if you're running a mailing list service, the usual case is that it's spam. A MAIL FROM rejection can have more innocent explanations, including a misconfigured MTA on the other side, but it is still more of a danger sign than a RCPT TO rejection.

(A significant volume of RCPT TO failures is still a danger sign, in part because it means that either the list of addresses is old or that the mailing list was badly maintained before it moved to your service. And if a mailing list has a few good mail-outs and then suddenly its RCPT TO failures spike upwards significantly, well, that's a bad sign itself. It could be that a whole bunch of user accounts just coincidentally got expired or filled up, but it's more likely that a bunch of anti-spam systems that reject at RCPT TO time suddenly woke up.)

Of course, all of this presumes that you are trying hard to run a 'clean' mailing list service instead of any of the various alternatives. I'm not convinced that there is or can be any such thing these days, as convenient as it would be for modern web applications if there was.