У меня та же проблема. http-линк в заголовке List-Unsubscribe, как я понимаю, не является обязательным, достаточно указать mailto. В любом случае, он не помогает. На гугловском блоге пишут:
This only works for some senders right now. We're actively encouraging senders to support auto-unsubscribe — we think 100% should. We won't provide the unsubscribe option on messages from spammers: we can't trust that they'll actually unsubscribe you, and they might even send you more spam. So you'll only see the unsubscribe option for senders that we're pretty sure are not spammers and will actually honor your unsubscribe request. We're being pretty conservative about which senders to trust in the beginning; over time, we hope to offer the ability to unsubscribe from more email.
