There are currently 2 known issues with Outlook. These are both server-side issues that are not directly related to POP Peeper (ie. they can affect other clients as well).
1) When sending mail using an Outlook account via SMTP with Oauth2 -- Server returns 5.7.3 Authentication unsuccessful... #535
Solution: Use password login instead of Oauth2. See this article for detailed instructions:
https://www.esumsoft.com/news/outlook-s ... th2-error/
This problem was first reported around Feb 7. You can find more information about my research into the problem, and that it affects all SMTP/Oauth2-based email clients here:
https://www.esumsoft.com/Forums/viewtop ... 618#p40618
edit: It's been reported that the following issue is now resolved, after about 24 hours:
2) Starting around Feb 22, some users have reported a persistent error with IMAP -- User is authenticated but not connected.
It has also been found that other clients are affected by this; unfortunately there is no known solution/work-around at this point. This issue may be regional (Europe; new report in Canada).