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.