This problem has been going on for years and has never been fixed. Years ago I used a hotmail account, and later an outlook.com account and this problem would come and go over the years with no solution real solution. I also had issues with using an email alias that was never fixed by Microsoft - kind of annoying as email aliases have been part of email standards virtually since the start of the internet and RFC's. I now use a third party email provider to resolve the issue for my main business account, but though outlook has been working well for the last year or so, my outlook.com profile in outlook (the office app) is now constantly prompting me for the outlook.com account password though this has not changed and still works fine if I use the web browser to login to outlook.com (same password). Incoming mail to my outlook.com account inbox (in outlook application) still works but the synchronization of some inbox folders is stuck. I don't think MS have ever had a real solution to these problems or any real desire to fix the problem and I don't think they ever will. They seem to just hope that after buying another computer, removing and reinstalling everything, upgrading to a newer office version, or creating a new email account, somehow it will fix itself. I am a former specialist in MS systems and since the days of Technet where problems were documented, analysed and causes and solutions published we have lost the ability to get problems diagnosed and fixed. The process now is just to ignore the problems and turn out new versions that might magically be sufficiently different that the old bugs will not resurface. It frustrates the hell out of me that basic features such as mailbox delivery and offline synchronization seem to be so fragile. Maybe it is due to the ads that appear in every folder in the web interface for outlook.com not working properly with MS office. Regardless, I don't think we will see a fix this decade.