The error message I get on sending to outlook:

"Login to server smtp.office365.com with username 'my name'@outlook.com failed"

check in outgoing server(smtp) in account settings. My guess is you have more than one outlook SMTP entry and it is using one set for something other than oauth authentication.

Hi Matt, Thanks for your reply and consideration - appreciated. Within Thunderbird when on my outlook account under the 'outgoing server', it highlights and says "smtp.office365.com". When I go into edit of this , it does list my google account outgoing server "smtp.gmail.com", but that is not highlighted - the google account outgoing server is only highlighted as active when I am in my gmail account in Thunderbird.

If Thunderbird when I am in my outlook account, is somehow using my google outgoing server in error, there is nothing to show this is happening and can only conclude some sort of bug in Thunderbird i.f that is the case. Afraid am still at a loss if anyone has other thoughts -Thanks Much

I fixed this my going to my actual Microsoft Outlook Acount and under alias names I had the primary name as my google e-mail name and changed this to my Outlook e-mail address. Alos, I went into Thunderbird Outlook account and made sure my name box contained the same name as my outlook e-mail address, as it had other outlook address consisting of my name, a long set of numbers, followed by outlook.com. Very tricky Indeed to work and try this out - took me ages.