Is the account POP or IMAP? What are the settings (outlook.office365.com on port 995 or 993, SSL/TLS, smtp.office365.com on port 587, STARTTLS)? What is the security/AV app, and is it scanning the profile folder or outgoing mail? Using a VPN? Is two-step verification on the account?

IMAP, Port 993, SSL/TLS, outlook.office365.com, normal password;

Outgoing: smtp.office365.com, port 567, normal password, STARTTLS

No security/AV app, not using VPN, no two-step verification on the account.

Deleted, re-downloaded Thunderbird, didn't help; reverted to earlier Thunderbird version, didn't help.

IMAP, Port 993, SSL/TLS, outlook.office365.com, normal password; Outgoing: smtp.office365.com, port 567, normal password, STARTTLS No security/AV app, not using VPN, no two-step verification on the account. Deleted, re-downloaded Thunderbird, didn't help; reverted to earlier Thunderbird version, didn't help.

Is that a typo? The port should be 587, not 567.

Yes, it is a typo, my apologies. It is 587.

The settings are correct, so in the absence of any error messages, it's hard to see what's blocking the account. Try deleting the passwords from Preferences/Security/Passwords/Saved Passwords, restart TB, and enter the password when prompted. That sometimes helps. Check that the User Name for the incoming and outgoing servers is the full email address. Under Preferences/Advanced/Network & Disk Space/Connection, try a different proxy setting.

The first two did not work but the last suggestion "Under Preferences/Advanced/Network & Disk Space/Connection, try a different proxy setting. " worked!

Thank you so much!!!

Stay safe, Mary.