rejections
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
Seen this recently? 451 Message temporarily deferred due to unresolvable RFC.5321 from domain; see https://postmaster.yahooinc.com/error-codes This is Yahoo doing some extra work to identify that the 5321.From domain1 of the mail is acceptable to them. Yahoo are going (slightly) beyond what’s required for the return path to be valid in SMTP terms. SMTP just requires that the return path be syntactically valid – i.e., looks like an email address – and that it be deliverable. The basic DNS check you might do would be to check if the right-hand-side of the email address has an MX record2. So for a bounce address of bounces@email.example.com you’d check to see if email.example.com had an MX record. Yahoo want to also check that it looks like a legitimate address in another way, that the organizational domain of the right-hand-side looks legitimate. The organizational domain is what you might think of as a “domain”