I've recently added a new work email to my thunderbird. The work email uses smtp.office365.com as the server. Initially when I set it up it all worked lovely. Then a week later it stopped. Wouldn't let me log on and told me that my login/password details were incorrect but they were the same ones I could use to login to webmail so I knew they were correct. Some googling later I found out that MS needed more security so I changed authentication from normal password to OAuth2. Success - I managed to login and download my emails. But when I tried to send, it told me that it was 'connecting to Office 365..'. But then 5 mins later it told me that the server had timed out and nothing had been sent. Since then I've tried many permutations of settings but nothing has worked. I have occasionally got different error messages including: ''Sending of the message failed. Failed due to unexpected error 80004005. No description is available. The message could not be sent using Outgoing server (SMTP) smtp.office365.com for an unknown reason. Please verify that your Outgoing server (SMTP) settings are correct and try again.'' or ''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.'' Trying again just seems to waste time rather than sending the email. Things I've tried: changing SSL/TLS to startTLS. Every other authentication method available there (and closing and reopening TB in between each trial) Deleting and reinstalling the whole mailbox checking that there is a microsoft cookie in the cookie stash I've checked from the microsoft end that MFA is disabled (the microsoft guy said this could be a factor) I have seen online a suggestion that I clearing history (caches and cookies) might help although this seems at odds with the advice to check there was a microsoft cookie so I haven't done that yet. A guy from Microsoft support suggested deleting my thunderbird profile and starting again (!) which I'm reluctant to do... Any suggestions??! For clarity - I'm using Thunderbird 78.14 (64 bit) and it's telling me that it's up to date I have a mac mini which is running MacOS Sierra version 10.12.6 The current settings in my thunderbird for this account are as follows: incoming server (working): IMAP mail server; outlook.office365.com; port 993; SSL/TLS; OAuth2; Outgoing server (not working) : smtp.office365.com; port 465; SSL/TLS; OAuth2 Thanks all in advance!!