EnterraHost
Microsoft 365 does not migrate the way an ordinary mail provider does, and the difference is worth understanding before you plan anything. There is no IMAP copy here, so the tidy two-stage approach used everywhere else does not apply.
The order is reversed, and that is the whole thing
An ordinary migration copies the mail first and switches the DNS afterwards, so nothing is lost while the copy runs. Microsoft 365 is the other way round. You point the domain at us first, and the mail is imported from an exported file afterwards.
That is not a preference, it is a consequence of there being one import per mailbox. A second pass cannot be run, so the sequence has to be arranged so the single pass happens at the right moment.
Practically that means getting the DNS in place, export each mailbox to a file, and hand the file over. Do not export first and sit on the file for a week, and do not hand over a file before the domain points at us.
Getting the DNS right
Where you change the records depends on who manages the domain. If the domain is registered with us, your DNS lives in the toolbox and you edit the records there. If it is still with another registrar or DNS provider, the change happens in that provider's own panel, and you can either make it there or transfer the domain to us first and manage everything in one place afterwards.
The records to change are the MX records and the related ones that prove your domain may send mail. If those are not already in place, adding SPF, DKIM and DMARC covers the sending half.
Exporting a mailbox
Outlook on a desktop does the export. Under the file menu there is an import/export option, and from there you choose to export to a file and pick the Outlook data file format. Select the mailbox, or the folders you want, and save the file somewhere you can upload it from.
The wording moves slightly between Outlook versions, but the path is the same one.
If you are exporting many mailboxes, there is a bulk path through the compliance portal rather than doing each one by hand. It needs a role that not every tenant grants by default, so it is worth asking whoever administers the tenant before you plan around it.
Keep the batches small
Because each mailbox gets one import, a file that arrives corrupted or incomplete has to be re-exported and re-imported. A batch of a handful of mailboxes can be fixed the same day. A batch of several hundred cannot.
Small batches are not caution for its own sake here. They are the difference between a problem that costs an hour and one that costs a week.
What moves, and what does not
Mail moves. Rules and filters are tied to the profile rather than the mailbox, so they are not part of the import, and moving your existing mail across covers carrying them over separately along with the rest of what does not travel on its own.
If your team uses Teams, SharePoint or document co-editing, check what you actually rely on against the tiers before you plan the move. Mail on its own is the simple half. The business email page sets out what Lite and Pro each include, and you can mix them under one domain if only some people need the extra.