550 5.7.1 — This message is likely unsolicited mail (Gmail)
Retrying will not help. The message will not be delivered.
The reply
550 5.7.1 Our system has detected that this message is likely unsolicited mail.Gmail decided the message looks like spam and refused it outright rather than filing it in the spam folder. This is a content and reputation judgement about this message from this sender, and unlike most rejections it comes with a real diagnostic: Postmaster Tools will show you the reputation behind it.
What it means
A permanent rejection at the policy stage, after the message was read. Google weighs the sending IP's reputation, the domain's reputation, the content, the links and how recipients have treated your mail before. The reply names no single reason because there rarely is one — it is a score crossing a line. The important consequence is that the recipient never sees this message at all, not even in spam.
Why it happens
- Domain or IP reputation has dropped — check both in Postmaster Tools, where they are shown separately.
- Recipients have been marking your mail as spam. This is the heaviest signal Google uses and the one Postmaster Tools reports directly.
- Content resembling bulk mail: one large image and no text, heavy promotional language, a link-shortener, or a body pasted from a template people abuse.
- A sudden rise in volume from a sender Gmail had seen sending little.
- Mail to addresses that never opted in, including a list bought or imported from elsewhere.
- Marketing and transactional mail sharing one domain, so a campaign's reputation drags the receipts down with it.
What to do
- Set up Postmaster Tools for the sending domain. It is free, it takes a DNS record, and it turns this from guesswork into two graphs: domain reputation and spam rate.
- Get the user-reported spam rate under 0.10% and keep it there. Google names 0.30% as the level that must never be reached.
- Split marketing from transactional mail onto separate subdomains so one cannot sink the other.
- Add List-Unsubscribe with one-click. People who cannot find the unsubscribe use the spam button, which costs far more.
- Remove subscribers who have not opened anything in months — unengaged recipients are what turns a good reputation bad.
- Send the same content from a different, established domain to separate a content problem from a reputation one.
Questions
Nothing about my mail changed. Why now?
Reputation is a moving average, so it crosses the line on an ordinary day rather than the day something changed. Postmaster Tools will show you the slope, which usually started weeks earlier.
Does the recipient see it in spam?
No. This is a rejection at SMTP time — the message was never accepted, so there is nothing in any folder. Telling the recipient to check spam will not help.
How long does it take to recover?
Weeks of clean sending, not days. Reputation is built from a trailing window, so the fix has to stay in place long enough to move the average.