There's this myth that automated spam detection is hard because spammers are all very clever masters of disguise.

No. Spammers are stupid as a shoe. They have dog shit for brains.

Automated spam detection is hard because the line between spam and "legitimate" marketing activity is a fiction.

@danslimmon
If that's the major difficulty, they could just classify "legitimate" marketing email as spam, and the problem would be solved.