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Right now I have a single mail server for IMAP. Let's say I want to introduce Exchange but not all users will be on it. Some users will be on my "legacy" IMAP, others on the "new" Exchange. Is it possible to "split up" your users (from the same e-mail domain) on two services like this? What would the MX records look like? My guess is that this isn't possible, but thought I'd ask.

By the way, I realize that Exchange can offer IMAP and all that, but my question is more about splitting users across services and the MX records. The actual protocols above are only examples.

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migrated from stackoverflow.com May 19 '10 at 17:55

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You can have more than one MX record, which point to different mail servers. There's a priority setting, so mail is attempted to be delivered to the primary first... –  OMG Ponies May 19 '10 at 16:53
    
@OMG Ponies Yes, but it's not going to try the second server on the list if you successfully connect to the first one, even if the first server rejects the message as a bad recipient. –  ewall May 19 '10 at 17:45
    
@ewall: Ah yes - my net/sysadmin skills are very rusty. –  OMG Ponies May 19 '10 at 17:52

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up vote 7 down vote accepted

There's no way you can do this via the MX records within a single domain name--DNS is always going to point emails destined to alice@acme.com and bob@acme.com to the server that answers at the highest-priority MX record. (In other words, the second -highest priority MX record is not used unless there was no response at the first IP address. If you connect to the first but it rejects the email as a bad address, the sending client is not going to re-try sending to the next MX record.)

However, you could do this with a subdomain. For example, your main email server (and MX record) would respond for all emails destined to acme.com, but your Exchange server would only respond to emails like charlie@exchange.acme.com. And one trick you could do to make this a little easier would be to have the main server except emails for charlie@acme.com and forward them all to charlie@exchange.acme.com. (You would want to configure Charlie's email client with a "Respond-To" address of charlie@acme.com, to make it less confusing for the senders.)

Also, there's a big caveat with the above trick: since Exchange is so tied into the Active Directory domain, it's not easy to move users to subdomains or change the subdomain that the Exchange server is answering to... so it may end up easier for you just to convert everyone off the old server to the new Exchange server all at once.

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It's pretty easy to setup Exchange to answer to a subdomain, or a variety of configurations. It does require AD though. Regardless, +1 you can't accomplish what he wants with MX records. –  Chris S May 19 '10 at 18:01

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