Say you have the ff. email server:
server1.com
has the mailbox foo@example.com
server2.com
has the mailbox bar@example.com
If the MX priority settings are:
Priority: 0
Destination: server1.com
Priority: 10
Destination: server2.com
Will sending an email to bar@example.com
eventually arrive at server2.com
or will it fail immediately because it doesn't exist in server1.com
?
If
server1
is accessible and if it answered so it could be interpreted thatbar@example.com
is permanently undeliverable (error code5xx
), 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 code4xx
(temporary error) forbar@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 toserver2
. The behaviour forfoo@example.com
in this case is the same.