In the old days, we used to accept email sent to any local account. This meant that various system accounts would collect outside mail instead of bouncing it. No one was reading, say, firstname.lastname@example.org, or email@example.com, but the mailboxes were there.
Enter the dictionary attacks. An awful lot of those standard accounts are three-letter names—rpm, gdm, bin, adm, etc. Spammers trying to guess addresses made up of three initials landed on these addresses, confirmed them, and added them to their lists. The system accounts began collecting spam.
Eventually we locked things down so that only “real” accounts would accept mail from outside. But here was this steady stream of 100% spam we could use to help train our filters.
The funny thing: these days, nearly all of it is for sex-related drugs or body part enlargements. Sent to software!
(Incidentally, if you can read this sentence, don’t send mail to firstname.lastname@example.org.)