Is there a better forum to field this question?

The correct server name for the outgoing server is smtp.office365.com. https://support.mozilla.org/en-US/kb/microsoft-oauth-authentication-and-thunderbird-202#w_your-outlook-or-hotmail-password-no-longer-works-with-thunderbird-and-you-cannot-send-or-receive-email

Hi christ1, I'll give that a try & Thanks!

Hi christ1, I'll give that a try & Thanks!

Did it work?

No. "Login to account "Microsoft 365" failed"

And asked if I wanted to: Retry Enter New Password Cancel

I clicked on "Enter New Password" and it timed out.

New box: Send Message Error Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.office365.com timed out. Try again.

I tried again with the same failure messages.

Try to delete the password/OAuth token Thunderbird has remembered for the account: At the top right of the Thunderbird window, click the menu button ≡ > Settings > Privacy & Security > Saved Passwords > Show Passwords

Also, please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button ≡, then select Help > Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.

The error 'Retry, Enter New Password, Cancel' appears when the authentication is 'normal password', not OAuth2, which prompts with a browser-like window. I suspect you have the account set to send on an smtp that is different from the one with smtp.office365.com, 587, STARTTLS, OAuth2, email address. Select the account on the left pane of Account Settings, then look at the Outgoing Server (SMTP) selected in the right pane.

Other possibilities are that smtp hasn't been enabled in O365 settings, plus an antivirus or VPN is interfering. But I think the issue is incorrect smtp selection.

I've tried both suggestions from christ1 and sfhowes and nothing has changed. I close Thunderbird and then reopen it. It asks if I want to "send your unsent messages now?". I click - Send Now. I get the same pop-up failure box. When I click on "Enter new password" I get the familiar "Send Message Error" ... smtp.office365.com timed out. Try again. Sorry but totally lost at this point.

Post a screenshot of the 'pop-up failure box'. Also, click Edit SMTP server... in Account Settings when the account is selected in the left pane and post a screenshot (as in the attached picture).

Hi sfhowes, I'll add the two screen shots as soon as I can but will be a bit later.

Thanks

Hi sfhowes, Actually have three screenshots.

The two failure messages occur after I open Thunderbird with an email that hasn't been sent. when I hit "send now" or whatever, I eventually get the 1st message. When I hit "enter new password" or whatever, I get the second message.

Thanks

The failure messages are out of order, below my text, but I'm guessing you can figure that out.

Remove all entries from Saved Passwords in Settings/Privacy & Security, and in the same section accept cookies, then restart TB and enter the account password in the OAuth2 window to allow TB access and receive mail. The oauth:// token that is then stored in Saved Passwords should be automatically applied when you try to send.

Are you using a profile in 136 that was used in an older version?

Hi sfhowes, I'll try the steps that you mention and answer the version question tomorrow.

Thanks