Failures vs Deferrals

There are 3 main reasons why an outbound message would fail:

• The domain to which you are sending does not exist (and this is confirmed by the DNS server).
• The message has been in queue for the full length of the retry period and has thus Expired.
• The remote server returned a 5xx series error code.

There are 3 main reasons why an outbound message would be deferred.

• The MTA could not connect to the domain to which you are sending.
• The MTA could not get DNS information for the send-to domain.
• The remote server returned a 4xx series error code.

Delivery Error Messages and What they Mean lists some of the most common error messages encountered.