Gmail error 550-5.7.1 "message blocked" — causes and the realistic fix path

Short answer: this is a permanent rejection, one level more serious than a rate limit. 550-5.7.1 ("this message is likely unsolicited mail" / "message blocked") means Gmail classified the message as spam at SMTP time and refused it outright — it never reached a spam folder.

Diagnosis: which variant are you seeing?

• "Likely unsolicited mail" — content + reputation verdict. Most common for cold outreach after sustained low engagement or complaints.
• "Does not meet IPv6/authentication requirements" — mechanical: your SPF/DKIM/PTR setup fails Gmail's sender requirements. The easiest variant to fix — grade your domain now.
• "Blocked for spam-like behavior" — pattern-based; often follows repeated 421-4.7.28 warnings that were ignored.

Recovery sequence

1. Stop sending to Gmail from the affected domain immediately. Every further 550 deepens the classification.
2. Fix the mechanical layer first: authentication (SPF, DKIM 2048-bit, DMARC with alignment), reverse DNS, and the blacklist check.
3. Audit what earned the reputation: list quality, complaint rate vs Google's 0.1%/0.3% thresholds, content patterns.
4. Re-warm slowly to engaged recipients only — replies and opens teach Gmail's models the mail is wanted. Weeks, not days.
5. If the domain is burned (550s persist after genuine cleanup + a month of good behavior), moving outreach to a fresh, properly-warmed domain is often cheaper than rehabilitation — this time with monitoring so drift never accumulates unseen.