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 From a legitimate mail (not even marketing) that I intentionally subscribed to:

X-Spam-Status: Yes, score=5.323
tests=[DIRECT_LOW_CONTRAST=2.499, DKIM_SIGNED=0.1, DKIM_VALID=-0.1,
DKIM_VALID_AU=-0.1, DMARC_PASS=-0.001, HEADER_FROM_DIFFERENT_DOMAINS=0.25,
HTML_FONT_LOW_CONTRAST=0.001, HTML_MESSAGE=0.001, MIME_HTML_ONLY=0.1,
MISSING_HEADERS=1.021, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001,
REPLYTO_WITHOUT_TO_CC=1.552]