Linda
Thanks for your response, but unfortunately that didn't help much. I've researched the migration options extensively and have all the information you provided. Let me ask the questions a bit differently, and see if you can help me further.
I first researched using cutover migration, staged migration or Hybrid as it appears those are the most robust and effective. But, we currently use hosted Exchange from www.123together.com and not on-premises Exchange. So I discounted those options, as I believe we need access to the Exchange server. Can you say whether any of these options should be viable when our existing Exchange is hosted with this other third-party?
And because I discounted those approaches, the only remaining approach is IMAP, and I have that working following all the instructions. BUT.... I'm getting a lot of skipped messages on a few mailboxes, and it's not because I've reached the 500,000 message limit or because all those messages are greater than 35MB. So there seems to be some other kind of issue, and I was wondering whether anyone has seen similar problems. Maybe my origin OST files are partially corrupt, or something like that?
So now I'm looking at the option of exporting the existing OST file (via Outlook) to a PST, then importing that again (via Outlook) to the new Office 365 account and letting it sync up to the cloud. On one level, this is better than IMAP anyway, as it includes calendar, contacts, tasks and notes, but on the other hand it requires every individual do this on their computer - which is not ideal.
Thanks
Mark.