Ask HN: How do you migrate Email and Exchange accounts

Hey guys, Im looking for tools to use in order to migrate a small amount (around 50) mailboxes from an MS Exchange hoster, to Office 365.

Most of the hard part has been synced already, but now iam at the point where i have drift between the "old" and "new" mailbox because the migration took way longer than expected.

My Ideal software would probably run in a container, but my main concern is that the tool needs to either run with a (modest) cloud fee or at least on linux so i can configure it myself. I dont have a problem with paying for good software, but as the org im representing right now is german, the tool needs to adhere to GDPR too.

What are you using? And why?