Gmail 550-5.7.1 Message Blocked as Likely Unsolicited Mail

What’s actually happening

This is a hard bounce — 550, message rejected, gone. Gmail decided the mail looks like spam and refused it at the door, then pointed you at the UnsolicitedMessageError help page. It is not an authentication rejection; that's the separate 5.7.26 error about failing SPF/DKIM/DMARC. Here the headers can authenticate perfectly and Gmail still blocks on reputation and content. It often hits some recipients and not others, and tends to flare when you push volume or send from a fresh IP.

Common causes

• Poor sending-IP or domain reputation in Gmail's eyes — the single biggest driver, visible in Google Postmaster Tools.
• Spammy content signals: link shorteners, sketchy attachments, image-heavy/text-light bodies, deceptive subject lines.
• The IP or a domain in the message is on a blocklist Gmail consults (Spamhaus and similar).
• Sending without real opt-in — cold lists, scraped addresses, or recipients who never asked to hear from you.
• A volume spike or unwarmed IP that makes the traffic pattern itself look like a spam run.

How to fix it

1. Open Google Postmaster Tools and read your reputationVerify your domain at postmaster.google.com and check the IP Reputation, Domain Reputation, and Spam Rate dashboards. 'Bad' or 'Low' reputation is your answer. Keep the user-reported spam rate under 0.1% and well clear of 0.3%, which is where Gmail starts blocking hard.
2. Confirm SPF, DKIM, and DMARC all pass and alignThis isn't the 5.7.26 auth error, but weak authentication still drags reputation down. Make sure SPF passes, DKIM signs with your domain, DMARC aligns, and bulk senders meet Gmail's one-click List-Unsubscribe (RFC 8058) requirement.
3. Check every relevant IP and domain against blocklistsLook up your sending IP and the domains in the message body on Spamhaus and a multi-RBL tool. If you're listed, follow each blocklist's delisting process and fix what got you listed before requesting removal.
4. Clean up the content and the listDrop link shorteners and risky attachments, fix the text-to-image ratio, and write honest subject lines. Remove non-opt-in addresses, hard bounces, and dead contacts — Gmail's filter weighs engagement heavily, and mailing people who never engage is what trips it.
5. Slow down and warm the IPIf this started after a volume jump or an IP change, throttle back and ramp gradually over days. A sudden surge from a low-history IP looks exactly like a spam campaign to Gmail's model.