The usual hotmail/live/outlook.com outgoing server is smtp.office365.com on port 587, STARTTLS security, authentication = normal password, User Name = email address. Do you have 2-step verification on the account or connect through a VPN?

Thanks for that, I seem to have resolved the issue!

Hmmm, still getting the same problem.

I've attached screenshots.

"Do you have 2-step verification on the account or connect through a VPN?" - Definitely not VPN, how do I know if it's a 2 step verification?

Thanks again!

If you have 2-step, it will be shown in the webmail's account settings. The server definitely supports STARTTLS, but since this is a domain account instead of a regular outlook.com account, it may be an Exchange account, or require SMTP to be enabled; check with your webhost. Also, you might have a security app that is interfering with the connection.

Thanks for the feedback.

Yeah somethings not right. I just had tech support on the phone, from Microsoft 365 - and we got it going on "Mail Bird". But the same settings still didnt work for Thunderbird :-(

Sucks, but appreciate your efforts.

I believe Mailbird has built-in support for Exchange accounts, whereas TB requires an add-on such as Owl.

Thanks for the feedback. Yeah somethings not right.

It sure is, you are still using startTLS instead of the correct setting. Change that to SSL/TLS. It is not the same thing as STARTTLS. Even if the Microsoft people do not understand that from what you have said.