I manage (very part time) an Office 365 instance for an NGO. When we set up the Office 365 back in 2016, we changed to a new domain name and migrated all mailbox content from their previous domain provider. However, the important task of decommissioning the legacy domain was never completed. Three years later and I find that some users still monitor their old e-mail addresses for mail.
I need to shut down the legacy e-mail service. Apart from the messiness, it is a paid service.
I'm sure this is not a new question but a search did not turn up a specific answer, so is this the right approach:
Add the legacy domain to the Office 365 instance
Allow Office 365 to manage the DNS, which will point the MX records to Office 365
Add e-mail aliases for the old domain addresses to all users still receiving e-mail on the old domain.
Yes, this will work as you think it will. Any new emails will be redirected to O365 so that users will receive them. I've done this myself with an old domain I allowed to lapse and picked up again a few years later; I still the occasional newsletter sent to the old domain.
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GeoffBartlett (Occasional Contributor)