This is an explanation for the email that goes in retry or failed delivery.
If we unable to connect to the recipient mail server, i.e. a connection times out or is refused, we immediately fail over to the next route that we have for the recipient domain. If there are no more routes, the mail is queued for delivery at a later time.
If we are able to connect to the mail server and receive a 4xx temporary error, we queue the mail and attempt to deliver it at a later time to the same server.
If we are able to connect to the mail server and receive a 5xx permanent error, we bounce the mail back to the sender.
Emails only go into retry and queue in the following circumstances:
Emails do not queue in the following circumstances: