Outlook does not differentiate between login username and email address / mailbox.
When using IMAP, SMTP, and POP3 based servers, email/mailbox and login username can be two different values.
These two values are usually harmonised in order to make configuration easier and to standardise user names but there is nothing stopping a login from having a primary mailbox ID which is different to that login name.
In short, "login user name!==from email address". There needs to be an easily accessible option to independently set both of these values at time of account creation, as there was in all previous versions of Outlook.
Jakob Wolf commented
make sure outlook is closed (via taskmanager), then press win+r and type in outlook.exe /manageprofiles. This worked for me. Still this change makes no sense at all.
M. S. commented
Same problem here.
Cannot you some of my mail account with Outlook because of this stupid change.
Several IMAP accounts can be used anymore because of this. Very annoying!
neither will it work with an IMPA server running on Synology
I simply cannot use Outlook with my university account anymore because of this issue.