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We have an on premises mail server (not exchange) and we have been evaluating Office 365 for a while and really like it.

As a migration path, and to work out any kinks, is it possible to have our current mail server hosting the majority of our users, while we slowly migrate some users (like our technical staff) to Office 365?

Will MX records allow for Office 365 to be queried for the user first, and then if not found, tries the next mail server on the list (our on premises server)?

Thanks!

Brian
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1 Answers1

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It depends on your mail server.

The way Exchange does this is by configuring its SMTP domain(s) as "internal relay", which basically means "try to find the recipient in this system and if it's not found route the message somewhere else", while instead the standard configuration of an "authoritative" domain is "try to find the recipient in this system and if it's not found reject the message".

Some mail servers other than Exchange can be configured this way (I did it personally with MDaemon), so yes, it might be possible, depending on which mail server you're using.

Community
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Massimo
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  • We are using Kerio Connect 8.4 – Brian Jun 03 '15 at 15:46
  • I don't personally know it, so I can't help you here. But you can ask a more specific question, just like I did with the MDaemon one. – Massimo Jun 03 '15 at 15:47
  • This helped a lot, I found a reference in their manual for "Domain Forwarding" Thanks for helping me steer in the right direction. – Brian Jun 03 '15 at 15:49
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    BTW, in that case I did the reverse: I switched the MX record to Exchange Online, and then configured Exchange Online to use an internal relay domain, thus sending to MDaemon all messages it wasn't able to deliver to its users; the tricky part was configuring MDaemon to to do the same while also avoiding mail loops. – Massimo Jun 03 '15 at 15:49
  • I was just going to bring up the situation that both would have to have forwarding so that users on each server could communicate with each other. – Brian Jun 03 '15 at 16:03
  • @Massimo, why was it tricky? – Henno Mar 10 '20 at 18:34
  • @Henno I don't remember the exact details after three years, but the main point is that if neither system is fully authoritative for a given SMTP domain and each one sends to the other the messages it can't handle internally, if a message is sent to a non-existing recipient it will enter an infinite loop. – Massimo Mar 10 '20 at 19:33