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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?

IMB
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  • Short answer: no. You need your MTA to route their messages to both local and remote MDA. In postfix, that's probably in your `transport_maps`. – SYN Feb 15 '21 at 15:40

1 Answers1

2

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.

Nikita Kipriyanov
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