Notes

After doing a cross-forest migration (see here), the Outlook 365 of one of our users behaved strangely. We faced the following phenomenona:

• Sent items was empty, although cache mode was enabled and the cache was up-to-date
• Address auto-complete didn't work
• Mails got stuck in Outbox, but could be sent by clicking on the Send/receive button (Outlook was configured to send immediately)

Outlook stated that it was connected to Exchange. What made this even more strange was that Outlook didn't always behave like this, mostly after every second start.

I tried several things. I removed all Outlook data from %appdata% and %localappdata%. Didn't work. I removed all Office-related data from the registry. No effect. Quick Repair did nothing. Even Online Repair didn't fix this behaviour.

As a last resort, I used Microsoft's uninstall tool (Office scrub) to completely remove Office and do a fresh installation. And this finally fixed the user's issues. I have no idea what was wrong, but I'm glad I could fix it.