Different email acount on different MX priority

Say you have the ff. email server:

server1.com has the mailbox [email protected]

server2.com has the mailbox [email protected]

If the MX priority settings are:

Priority: 0
Destination: server1.com

Priority: 10
Destination: server2.com

Will sending an email to [email protected] eventually arrive at server2.com or will it fail immediately because it doesn't exist in server1.com?

Solution 1:

If server1 is accessible and if it answered so it could be interpreted that [email protected] 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 [email protected], 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 [email protected] in this case is the same.