I've got a bit of an issue with Office 365 - after migrating an user from an existing IMAP account it seems like their "Archive" folder conflicted with the default one provided by O365 and as a result their existing archive is now named "Archive_0".
I would like to merge this with the main "Archive" folder but the issue is that we're looking at several thousand items - so much that Mac Mail is struggling to even load it (initial download took a good 10 minutes).
Is there a way to do it server-side, though some kind of API? I'd like to do it as a batch operation instead of downloading/uploading every mail which would take ages. Preferably I would like to use my Mac but would be able to install a Windows VM if necessary.
Regards.
You can export the content to PST, using the eDiscvoery tools. Then re-import the mail thru the Office365 Import service.