can you download from this link https://addons.mozilla.org/en-US/thunderbird/addon/exquilla-exchange-web-services/?src=ss

Hi Matt, Thank you so much for your insights and guidance. I followed the instructions given, and as far as I can tell, everything loaded properly. When I go to send emails, the window "Connecting to Office365.com" times out and I am unable to send anything out. I am a geezer and don't understand all of what I am doing. Walt

Office365 supports IMAP and POp. Perhaps you should try them first as they require no add-on

See https://support.office.com/en-us/article/Outlook-settings-for-POP-and-IMAP-access-for-Office-365-for-business-or-Microsoft-Exchange-accounts-7fc677eb-2491-4cbc-8153-8e7113525f6c

Hi Matt, Thank you again for your advice. 1) I am able to receive emails, I am unable to send. Was advised that would be a problem with SMTP (whatever that is). I put in smtp.office365.com to port 587 as instructed. It still says default 465 to the right of the window. Need I change default or do something else?

2) I have been using Thunderbird for probably 15 years and all of my addresses, etc are on it. If necessary, is it possible to back up all of that info an import it to maybe applemail if I can't get this to work?

Thank you for all of your assistance and insights. I'm sorry to be such a bother. Walt

I found a different solution to the problem. Thanks for the help. I am finished now, so we may close this out. Walt