How to migrate e-mail addresses from an old to a new domain, but allow to still send to the old e-mail addresses?

Question

Our company currently has a domain old.example.

Going forward, they want to use a new domain new.example as primary domain.

1. This means that we will host the new website on new.example and next, set a redirect from old.com to new.example. No problem here.
2. Going forward, we will make new e-mail addresses like mail@new.example.
3. My question is about dealing with the e-mails accounts already linked to old.com. We would like to make the old e-mail addressess available via mail@new.example. However, when somebody still mails to mail@old.example, the mail should still be received on mail@new.example.

Mailboxes will be hosted on Office 365. I will add both the old and new domain to Office 365.

How can we achieve this? Should we re-create all e-mail addresses with the domain new.example & then create a forward rule from mail@old.example to mail@new.example. There has to be a smarter way to do this? Some kind of alias method which can be used?

Answer 1

the process at https://support.office.com/en-gb/article/add-or-remove-a-user-s-email-alias-cb86b53b-bebf-4e0a-b781-c49270d85ea3 can be used, with the old address as the alias and the new address as the primary. a précis of the process is replicated below to ward against link rot:

Add email aliases to a user

You must have admin permissions in Microsoft 365 to do this.

• Sign in with your Microsoft 365 admin account on the Microsoft 365 admin center.

• Select Users.

• On the Active users page, choose the user.

• Next to User name / Email, choose Edit.. Note: new users can take 30 minutes to fully provision

• On the Edit email addresses page, in the text box under Alias, type the first part of the new email alias and picks domain, then choose Add.

• choose Save.

Wait 24 hours for the new aliases to populate throughout Microsoft 365.

The user now has a primary address and an alias. All mail sent to either address goes to their inbox. The primary address will be the From address when sending new mail

Everyone has a "real" mailbox of the new domain, and an alias attached to it of the old domain (o365 knows of both domains). Mails will deliver to the new boxes and replies and new mails will go out with the new domain on so gradually the world will (should) convert to using the new domain.