Enrico Zini notes under a heading of "Spam is useful":

Really. I just realized that. In the past, if you wanted to test mail delivery on your mail server, you had to bother logging to a remote server and sending yourself a mail. Now that's not needed anymore: as soon as the server works, spam messages start coming in. So it's not spam, it's PING mails.

He's got a point there. I've been doing the same with my recent spam/virus reduction setup changes (switched to mimedefang and love it; more on that in another post).