Gmail 5.7.1: message blocked, and what to do about it
Gmailreputation_blockaction: pause
Gmail 5.7.1 is a hard block. Unlike 4.7.28, this is Gmail refusing the message outright, usually on content or on the reputation of the sending IP or domain. Retrying makes it worse. Stop sending to Gmail from that source and find the cause.
What it looks like in the log
550-5.7.1 Our system has detected that this message is likely unsolicited mail.
550-5.7.1 The IP you're using to send mail is not authorized to send email directly to our servers.
550-5.7.1 This message has been blocked because it is likely unsolicited.
Why it happens
IP or domain reputation has degraded, most often after a complaint-rate rise.
Content or a linked domain is being scored as spam, including URLs you do not control.
Sending from an IP Gmail treats as residential or dynamic.
A shared IP damaged by another tenant on the same range.
What to do
Stop sending to Gmail from that IP or domain. Continued retries deepen the problem.
Check Google Postmaster Tools for domain and IP reputation and for the spam rate. Under 0.3% is the threshold that matters.
Separate the variables: if only one range is affected, it is reputation. If every range is affected, it is content or authentication.
Clean the list before resuming. Reputation recovers through good sending, not through appeals.