Resolved: An unknown error has occurred in outlook. mail could not be sent. the message has been moved to your drafts folder.

Issue: an unknown error has occurred in outlook. mail could not be sent. the message has been moved to your drafts folder. 

Issue detail:

User A has full access on user B mailbox. have migrate User B mailbox to Exchange 2019 recently, but User A was still on Exchange 2013. After that the issue was started (User A was unable to send email from User B mailbox, of which he has the rights)

Before migration of user B mailbox, there was no issue, User A was able to send the emails from User B.

Solution:

Did two things to resolve this issue:

1. migration User A as well to exchange 2019
2. Assign the permission again on User B to user A.