Rated 5 out of 5 stars

Came back to upgrade my review to 5 stars. After the Thunderbird stuff-up my Office account eventually reappeared but Sent emails were disappearing. Emailed a query to the developer who replied within hours to look in Account settings>Folders and Copies. (I had wrongly been looking for a fix in Sent Items>Properties). Sure enough, sent items were getting stored in the native Thunderbird 'Local Folders' sent folder. Redirected that to my Office sent folder and all is good again.

This review is for a previous version of the add-on (1.2.3). 

I'm happy that your problem is fixed!