I have various IMAP accounts which all work fine both from Outlook on Windows and from MailDroid, Thunderbird, Samsung Email, Windows 10 Mail, etc.
One particular email address gives a login error every time on Outlook for Android, both via autodetect and with manual input, despite my entering exactly the same values as on Outlook for Windows etc. (servers, user names, passwords, SSL/TLS port numbers).
I tested the setup against the Microsoft IMAP tester here https://testconnectivity.microsoft.com/tests/O365Imap/input and it passed. In particular there are no material SSL certificate issues.
The account works fine with Outlook for Android on a brand new Android installation (BlueStacks) but not on my 'phone or tablet.
I have also tried (against my better judgment) using the non-SSL ports (which the email server supports) - same login error experienced.
(edited to remove superfluous/confusing material)