Google / SMTP error code 421
421 4.7.28 [x.xx.xx.xx] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. - gsmtp
High severity
Gmail has determined your messages are unsolicted, so messages sent over this IP may be temporarily rejected. See Google's documentation for best practices on sending to Gmail to avoid this.
Documentation
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