You mentioned smtp.mail.outlook365.com - see if that is what you have, as that is the wrong server spec. If you have two versions, you might check to see where they are installed. One of them might be the MS version, which is installed in appdata\local. The thunderbird version would be in program files (x86). I suggest removing one of them, but be careful to not lose the appropriate profile.

Hi David, thank you for your reponse.

Hmm... I will 'park' the two installed versions for the time being, and concentrate on server settings.

I changed the settings to match those on my desktop PC - but this has the latest Thunderbird version.

The saying "a little knowledge is a dangerous thing" is true for me.

I have reverted to incoming server imap-mail.outlook.com for each of my email accounts. Is this correct?

I have changed outgoing server settings too, but they are not correct because messages won't send.

Please tell me what these should be.

Thank you!

the incoming server is outlook.office365.com, port 993, SSL/TLS, Oauth2 the outgoing server is smtp.office365.com, port 587, STARTTLS, Oauth2

Oh! Its 'back to the drawing board' then. I am 'keeping my fingers crossed' everything will work when I have made these changes. Thank you!

Hello again David

Thank you very much for your help. I am extremely grateful.

Microsoft is asking me, in some cases repeatedly, to enter the password for email accounts - authentication failures when connecting to the server. I am hoping these instances will become fewer.

I cannot send from 3 (thankfully unimportant) email accounts. The mail server 'thinks' I am sending emails from one of my OTHER email addresses. I don't suppose you know why this is? (Its the same other email address for all 3 accounts.) This issue is only occurring on the laptop. I can 'live with it' if needs be.

Suggestion:

1. click 'account settings'
2. scroll down to 'Outgoing Servers (SMTP) and click there
3. for each SMTP entry, change the DESCRIPTION field to contain the email address for the account. Check also that the User Name field for that account is correct.
4. then, still within account settings, click the account name in left side for each account. On the right side that displays info for the account, look at bottom line where there is the link to 'Outgoing server SMTP'. What you see there should be what you entered in prior step for DESCRIPTION field. If not, then click the dropdown menu there and select the appropriate SMTP server.

Doing the above may correct your problem if sending with the wrong account.

David, your suggestion worked. Thank you very much!

You are very welcome. I was glad to assist. :)