[kooshball]

think about what the PM of antispam at Gmail is optimizing for. their job is to provide defense against adversarial spammers.

anything trivial for you to do to get into someone's inbox can also be trivial for spammers. they'll have a bot that do it 1000 times a day. Gmail can't stop spam altogether so the general strategy is to try to make it as painful and expensive as possible to deter the marginal spammers.

any sort of spam at scale is easier to detect. that's why most marginal spam happens with new IP. similar issue in the past with hacked Gmail accounts. this is where google is cracking down harder and why you're running into these issues.