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I'm told that it's best practice to have a secondary MX record pointing to a secondary mail server in case the main one fails.

If the main mail server fails, how will the users on the main server get their email? Do I need to mirror all user accounts on the secondary server?

I'm new to mail servers, so I'm not sure if my question makes sense.

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up vote 6 down vote accepted

Answer: it depends. Most secondary server setups just hold the mail until the primary comes back up, at which point the mail is either delivered or bounced.

If you have either a separate user database that both servers have access to (eg LDAP) or you regularly clone your account list between the servers, then you could probably configure a mailserver to act as a secondary (forwarding the mail to the primary server when it's back up) and only accept mail for valid addresses.

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This depends entirely on your mail server architecture. In an Exchange setup for example, you can have multiple edge/hub transports, multiple CASes, and mirrored mail databases using DAGs.

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