If server1
is accessible and if it answered so it could be interpreted that bar@example.com
is permanently undeliverable (error code 5xx
), the sending server should give up and don't try other servers. In this case it drops mail as undeliverable and optionally creates a non-delivery notification to the original sender.
If server1
answered with error code 4xx
(temporary error) for bar@example.com
, sending server should try to connect to another recipient server (server2
) and try to deliver mail to it. If it also throws an error, sending server may queue and retry both servers for some extended period of time (say, a week), and then it gives up and sends non-delivery to original sender. Also it may send intermediate DSN about mail delay.
If server1
is inaccessible, sending server has no other option than connect to server2
. The behaviour for foo@example.com
in this case is the same.