Microsoft Old Outlook desktop incorrect username or password error

We had a customer complain that her Microsoft application on her computer had started giving her an error that her Outlook email account’s username or password was incorrect.
I immediately suspected a 2FA problem since the old Outlook needs an app password when 2FA has been enabled.
I was able to log into her Microsoft account which proved that her credentials were correct. I noticed that 2FA had not been turned on.
Fearing that the Credentials cache on her computer had gotten corrupted, I deleted the stored credentials for her Outlook. Upon signing in with the same credentials, the error persisted.
I tried changing the mail servers from pop-mail.outlook.com and smtp-mail.outlook.com to outlook.office365.com but it didn’t help.
I then suspected that the Outlook profile had gotten corrupted so deleted the existing Outlook Profile and recreated it but alas the error remained.
I then chose the option to create a Microsoft 365 account rather than the legacy POP configuration.
This uses a Microsoft Exchange server in the cloud to handle the email account.
After doing that, her emails started downloading again.
It seems that Microsoft has removed the ability for the old Outlook programme to use a POP email account.