Office 365: Basic Auth is disabled
Since October 2022 Basic Auth is by default disabled for IMAP and POP3 in Exchange Online / Office 365. Microsoft recommends using OAuth 2.0. This is not a big problem for Mail.dll email client for .net users as it support OAuth 2.0 in full.
You should switch to OAuth 2.0 for authentication purposes:
Daemons/Services: Password grant (MFA/2FA must be turned off for this account):
Daemons/Services: Client credential flow:
Web apps (requires user interaction):
Standalone devices (requires very little interaction):
Desktop apps (requires user interaction):
Re-enable Basic Auth
For some time it was possible to re-enable Basic Auth for IMAP and POP3 for your tenant – it is no longer possible to do that.