Why on earth is it necessary to physically exchange tomes when that is not only the raison d'etre of Microsoft Exchange but remarkably the sub rosa of the actual article up top.
You don't have to answer, of course, respect the security, but genuinely puzzled.
I honestly don’t know - I’m a network guy, not an O365 guy (I’ve done SOME O365 work, but not a SME).
My understanding is that those splitting off are going to a completely different company (*@abc.com to *@xyz.com), and that entailed migrating *.pst files from one domain to another - so when everyone logged in Monday morning, opened Outlook, it started doing the okay - we’re taking everything from abc.com away, and making you xyz.com now.
¯\_(ツ)_/¯