Do you have these settings?

incoming: outlook.office365.com, 993 (IMAP) or 995 (POP), SSL/TLS, OAuth2, email address

outgoing: smtp.office365.com, 587, STARTTLS, OAuth2, email address

Yep. Looks that way. All the same.

What is the antivirus, and is a VPN in use?

I think this account has been hacked. I just received an email sent to me from me -I therefore cannot block it. What can I do with that. Change my email address within T-bird?

A message can be made to look like it's sent from you to you, but it's probably a scam or phishing attempt (examine source Ctrl+U). Perhaps access the account in webmail and change your password (which might not change the error message). After changing, remove old entries from Saved Passwords in Settings, restart TB. Antivirus? VPN?

AV = Microsoft Defender - No -- NO VPN -- so please clarify - access the account in web mail? Can't change the password within TB? Thanks. This hacker is asking for $$...

ok got it -- go to Outlook on line is what you mean, right? Change pw there.

Ok - changed pw.

After changing, remove old entries from Saved Passwords in Settings, restart TB. I don't see where to do this.

Settings/Privacy & Security, Saved Passwords, remove entries for the account, restart TB, enter new password in OAuth window to allow access.

Scam messages supposedly using your account, demanding payment 'or else', are a common scam.

I found this -- delete those?

Yes, delete them both. After re-authentication, there will be a single oauth:// entry that works for incoming and outgoing.

So delete incoming and outgoing as shown in clip. Put new one in as indicated.

You can't enter a new oauth:// entry manually in Saved Passwords - it will be automatically added when the authentication is completed.

OK--all set I hope. :-}

Yes - that is what happened. It put the new password in as you indicated.

That creep that accessed my account is why I couldn't send a msg right? I just did a test and now the send works. Whew...Thanks so much for your terrific well-explained therapy session...:-)

You probably weren't hacked, but TB was possibly sending the wrong entry in Saved Passwords. With OAuth2, imap://and smtp:// entries (passwords) are not needed, just an oauth:// token.

The reason I think it was hacked as this creep wrote a msg and said if I didn't pay he would spread the word about me - bad person ya da ya da....Isn't that a hack?

So if what you say is true, is that a glitch with TB? If passwords aren't needed, and this is a glitch, it may happen again?

You weren't hacked, just 'spoofed', as proven by your ability to change the password (unless your system is truly infected by malware). This has nothing to do with TB, and the only possible glitch is the obsolete entries in Saved Passwords. You might see these scams again, but generally they can be ignored. The sender preys on people's unawareness of such ploys.