If you need something to sync imap then you can use:

I used it to move from smartermail to hosted exchange.

Once you get past the learning curve, BitTitan is very good for this. Let's you simulate migrations and do multiple migrations for a given user. When you're ready to cut over, you do a final migration. About $10 per mailbox. Their support is via email; support is good, response time is OK, not always fast. - - Uncompensated endorsement, YMMV, as usual. 

Support from BitTitan, last time I tried it, was terrible. Gave up.

Tried EDBMails, not good at all with SM - skipped source folders without warning, and other issues. Most recently had good results with CodeTwo migrations. And the price was right, too. As with all these products, some learning curve is part of the process.

Wouldn't one of the best ways be for your target vendor to provide a migration tool?

Btw, we use ImapSync to migrate to/from SM.

I like IMAPSync and MigrationWiz by Bittitan. Both have always worked well for me.

Yeah, Microsoft 365 has a migration tool. It's not very friendly nor very granular. So we often go with 3rd-party tools. 

And yes, one would think that a behemoth like Microsoft would have a good, very flexible and free tool for migrations, seeing as how they want your business and all, but sadly, nope.