Variants of this question keeps showing up from time to time. It goes roughly like this:

I am migrating email for example.com from provider A to provider B. During migration I will have some users on provider A and other users on provider B. I created MX records for example.com pointing to both A's servers and B's servers. But it doesn't work.

The most recent such question.

I went to look for a canonical question with a suitable answer. I didn't find a canonical question, but I found two good candidates:

Should we appoint one of those two questions to be canonical?

It's a good question. But I'm not convinced we have a good existing question that covers this issue in sufficient depth:

  • I don't see that the existing answers fully explain what is happening.
  • The existing questions and answers seem to be MTA-specific.
  • Some MTAs and cloud email services don't seem to be covered at all.
  • None of the existing answers I saw provided workarounds, which ought to exist for at least some configurations.

Now if someone comes along and writes a good answer, or a self-answered question, that goes all the way and provides as much information as possible, then I'd happily vote for it and consider it as a duplicate target.

This would be more a question about shared SMTP domains (whose configuration strongly depend on the actual mail servers being in use) than MX records.

Multiple MX records, depending on priority and reachability, can cause incoming messages to go to a server or to another one... but there is no way all servers are going to receive a copy. They have to be configured to share the same SMTP domain and correctly handle messages regardless of which server actually hosts a specific mailbox.

  • Those are all aspects which would need to be covered by the answer to such a canonical question. Simply pointing MX records to the servers for both service providers won't work. But that doesn't stop people trying and asking here why it doesn't work. – kasperd Oct 28 at 22:05
  • The problem as I see it with such a question is, there is no simple universal answer. I am able to configure this with Exchange, but not with other mail servers. The only answer that will always apply is "having multiple MX records is not enough". – Massimo Oct 28 at 22:08
  • It's possible to give a generic answer covering most of the solution. There will be some part of the setup which will dependent on the specific mail server/provider. But it is possible to give a generic answer explaining what that service has to support to make such a setup possible. – kasperd Oct 28 at 22:14

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