google hates you
Gmail has very recently unveiled a series of new deferral/rejection messages. You’ll want to familiarize yourself with the following Gmail rejection messages: 421 4.7.28 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 421 4.7.28 Gmail has detected an unusual rate of unsolicited mail containing one of your URL domains. To protect our users from spam, mail with the URL has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. – gsmtp 421 4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your SPF domain. To protect our users from spam, mail sent from your domain has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our