i am successfully using this

https://imapsync.lamiral.info/

Quite intense and without problems

I have transferred about 600 emails

Second vote for IMAPSync. We've had mixed results with SmarterMail's tools, but IMAPSync is always perfect.

IMAPSync/IMAPCopy are definitely the way to go when completing bulk migrations. There is nothing better than handing it a list of usernames, and watching it go to work! :)

@Mark Smith - Why on earth would someone on a Smartermail forum be looking for a tool to migrate to Exchange 365?  

Side note-- we run Smartermail Enterprise, on-prem Exchange Server enterprise edition, and we manage Exchange 365 for customers.  Of the 3, our admins hate Exchange 365 the most.  It's a mess to administer, requiring a disjoint mix of marginally-useful web GUI and powershell.  It's dog-slow (compliance deletes take forever to run, changes take many minutes to sync between Azure & Exchange) and it's spam/phish filtering is stupidly aggressive, without a useful way to whitelist for more than 30 days.  I think the primary reason Exchange 365 has as many customers as it does, is the convenience factor for those who buy 365 subscriptions.  That, and because Microsoft made Exchange on-prem prohibitively expensive now.  From a functional and administrative standpoint, in our experience Exchange 365 sucks.