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I am at the point where everything has synced between my on premises exchange server and the office 365 migration batch. The next step is to modify my MX records so that mail begins to be direct to the 365 mailboxes. The following is from the cutover guide provided by microsoft:

It can take up to 72 hours for the email systems of your customers and partners to recognize the changed MX record. Wait at least 72 hours before you proceed to the next task: Delete the cutover migration batch.

My question is in regards to how end users should handle this 72 hour period. Should they have two profiles/accounts in outlook/apple mail, one pointing to the new inbox, one the old? Or can they simply remove the old and everything delivered to the old will be immediately synced to the new?

Tom Swifty
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2 Answers2

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I haven't done any O365 migrations, but here's what I'd say about it.

  1. The 72 hour "warning" is a bit of misinformation. It will take the amount of time related to the TTL of the MX record, whether that's 1 hour or 72 hours. It's based on the TTL, not some antiquated misinformation about DNS record changes. If the TTL for your MX record is 1 hour thenm it will take 1 hour, and only for those systems that already have your MX record in their DNS cache. For systems that don't have the MX record in their DNS cache, they'll find the new MX record immediately.

  2. I would set up your email clients to use the new Exchange Server. They can use webmail to access the old Exchange Server to retrieve/respond to any email that happens to wind up at the old Exchange Server. Once the cutover is complete you can export any straggling email from the old Exchange Server and import it into the new Exchange Server.

joeqwerty
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  • So with my mx record TTL set to 300, the only messages that might wind up in the old mailboxes are ones in that 5 minute period? – Tom Swifty Apr 07 '16 at 15:19
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    Give or take, yes. My point was that if the MX record isn't 72 hours then it's not going to take 72 hours. You should plan for some variance. – joeqwerty Apr 07 '16 at 15:20
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    @TomSwifty Note that if your TTL was *previously* set to a higher value, you have to wait for that to expire before the new 5 minute TTL is meaningful. – o11c Apr 07 '16 at 17:41
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The MX record will only affect mail delivery for your SMTP domain; client configuration is up to you and totally independent from it. When the MX change will be fully propagated, all mailservers sending messages to your domain will send them to Exchange Online, instead of your current mail server. That's it. Also, the 72 hours delay is related to DNS caching: if you modify the TTL for your MX record to 1 hour, the change will be propagated worldwide in at most 119 minutes.

Actual client redirection will depend on whether you have setup a hybrid configuration or not; if you did, then your Exchange will automatically redirect clients to Exchange Online when their mailbox is migrated. If you didn't, you'll have to reconfigure each Outlook client (manually or using a script or whatever).

Massimo
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