There is a particular scenario that is becoming more and more common it today’s modern workplace…the requirement to migrate Exchange Online mailboxes from one Office 365 tenant to another. This is an evolution of a requirement that comes out of acquisitions and mergers.
Traditionally, in an on-premises Exchange environment, mailboxes would be migrated between Exchange organisations (with a lot of preparation and connectivity) or they would be exported to a PST file and imported in to the new mailbox. These methods no longer apply in a cloud environment with mailboxes hosed in Exchange Online. Sure, there are options. You could migrate all mailboxes back on-premises and use the traditional methods mentioned. That would usually require the addition of new Exchange servers on-premises. Servers you worked hard to get rid of. There is also the option of third-party tools, however, they can be expensive. Especially if there is a lot of data to be migrated. You do also have the option to export to PST using eDiscovery tools in Office 365. But this is a horrible experience for both user and admin and one I would never recommend.
So, migrating mailboxes from one tenant to another is not so simple, unless you have cash to spare. However, there is soon to be a solution to all of this to make life easier.