We’ve acquired a company that uses Google Workspace, while we use Microsoft 365. We control the old email domain and plan to move people to our addresses. What’s a sensible way to handle incoming mail to the old addresses during and after the transition?
Should we forward the old company’s email or keep the domain receiving mail in Microsoft 365?
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v4portrail
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If you control the domain and want old addresses to keep working, I’d usually have Microsoft 365 receive mail for that domain directly rather than forward messages out of Google. Add and verify the domain in your tenant, put each old address on the right mailbox as an alias, then change the domain’s MX records when you’re ready.
Test a few addresses and mail paths before calling the cutover done. Keep the Google environment available until you’ve checked that mail and any needed historical data are accounted for.
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Think about whether the old domain is meant to stay active for months or for years. If you expect customers or suppliers to keep using it, accepting mail for it in Microsoft 365 and assigning the old addresses as aliases avoids relying on a forwarding rule or on the old Google accounts staying around.
If the goal is a short transition and you don’t want to host the old domain in your tenant, temporary forwarding can work. Set an end date, monitor for messages that still arrive, and make sure the right people update their contact details. Either way, preserve access to old mail separately from the plan for new incoming mail.
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Don’t leave the DNS authentication pieces until after the MX change. Plan SPF for the services that will send mail using the old domain, configure DKIM for Microsoft 365, and review DMARC so legitimate messages have the intended authentication and alignment. Check that the old domain’s DNS changes are in place before the cutover, and test both inbound and outbound mail.
Also confirm what each address should become. An alias is fine when it should deliver to an existing person’s mailbox. Addresses that need their own mailbox, separate history, or a distinct sending identity may need a different setup.
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Make a simple cutover checklist and tell users what changes, when they happen, and where to look for old messages. Include shared addresses and automated mail, not just individual inboxes. A quiet mail-flow test with a few internal and external senders can catch address mapping mistakes before everyone depends on the new setup.
Keep the old Google accounts available until migration and access checks are complete. You can then retire them on a planned schedule rather than treating the MX change as proof that everything has moved.
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v4portrail:
That sounds cleaner. We do want customers to be able to keep using the old addresses for a while.