I have a web app that uses oauth authentication with IMAP/SMTP protocols to access emails in office365/outlook mailboxes. But a few customers had a problem connecting through those protocols Server returns A0003 BAD User is authenticated but not connected after selecting mailbox. I've tested oauth flow with my personal outlook and office365 accounts - and I haven't experienced such error.
Customers that have this problem has a normal mailbox, not shared one. Also SMPT/IMAP options are turned on. The problem began a couple days ago, no changes in oauth flow in my web application were done.
I've did some research on my own but didn't find much. So my questions are :
what could cause this error and what actions needed to be applied to solve it?
is it from my end(oauth flow) or customer(o365/azure portals settings)?