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I'm trying to set up a Windows Server 2012 and a Linux Server (CentOS 7) to act as backup mail servers for each other.

They each host different domains. So lets say the Linux server is the host of domain1.com and has an MX record of priority 0

I want to have the Windows server be the backup mail server so I may have another MX record of priority 10.

So domain2.com may be the opposite, so the Windows server is primary and the Linux server is backup.

What would I have to do to make this work?

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  • Change the priority to be what you want. This doesn't seem difficult. Feb 8, 2021 at 15:01

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From the DNS perspective it is indeed as simple as you say: for each domain simply add a second MX record for the back-up mailserver and ensure that the priority value for the backup is higher than that of the existing primary mailserver.

But before you execute the DNS change: you need to adjust the settings of each backup mail server.
By default mail servers should be (and usually are) configured to reject all messages for domains that are not local to that mail server. You will need to explicitly configure the domain(s) for which it should act as back-up as authorized relay destinations. That means that the back-up mail server will accept, queue and eventually forward mail messages to the primary mail server, but only for that domain. (If you mess this up: your server will be found and used by spammers to send huge amounts of unsolicited email)

How to implement that depends on the specific mail server software you're using. See for example:

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