Shadow banning doesn't work against any adversary that's more sophisticated than an annoying person on the internet. As you observe, email spam filters are bas

Shadow banning doesn't work against any adversary that's more sophisticated than an annoying person on the internet. As you observe, email spam filters are basically just "shadow bans" and spammers aren't confused by them at all, they just send themselves test messages. Spam filters work for lots of reasons but not because spammers can't figure out if they're caught or not.