This is I think a bug report and warning to others, rather than a question. I recently completed migrating my own company and a client to Office365. By far the hardest part is migrating IMAP email archives - I tried several methods and lost 100's of emails in the process, before finding a slow but reliable solution (basically use third party software to export as an archive, reimport into Outlook and upload from there - how sad is that?).
[And the connected accounts feature does not work well at all]
However Office refuses to accept some emails - at least in Outlook 2011 for Mac, namely Delivery Failures and Read Receipts, which can end up in peoples archives and are quite slow to track down and delete.
The other nasty surprise was that every archived email that was originally sent with an attached read receipt request will trigger a new read receipt from the server. Some of my clients were bombarded with read receipts from emails they sent in 2009!
This was very embarrassing and I think a bug - why would Exchange generate a read receipt for an email I have imported from an Archive?
The solution is to turn off Read Receipts from OWA settings before starting to import email.