Use smtp.office365.com for the outgoing server, check that IMAP and POP are enabled for the account, check that cookies are accepted in TB settings, and disable the VPN if in use.

https://support.mozilla.org/en-US/kb/microsoft-oauth-authentication-and-thunderbird-202

Use smtp.office365.com for the outgoing server, check that IMAP and POP are enabled for the account, check that cookies are accepted in TB settings, and disable the VPN if in use. https://support.mozilla.org/en-US/kb/microsoft-oauth-authentication-and-thunderbird-202

Hi. smtp.office365.com is already the outgoing server. IMAP and POP are already enabled for the account I can used outlook on my mobile device fine (it's enabled for TB too). Cookies are accepted in TB.

Does the user name/email address in the 'From' section of your email (1) match the User name in your SMTP Settings (2) and the email address in your Server Settings (3)?

1 - Hamburger (top right)/Account Settings - click on the name of the account through which you are sending the email. On the right is the return email address called 'Email Address' unless you have put another email address in 'Reply-to Address'.

2 - Hamburger (top right)/Account Settings/Outgoing Server (SMTP). Highlight the server through which you are sending the email and the User Name is displayed at the bottom. It can be changed using the 'Edit' button.

3 - Hamburger (top right)/Account Settings - click on the 'Server Settings' under the account through which you are sending the email. The email address is in the 'User Name' field.

Does the user name/email address in the 'From' section of your email (1) match the User name in your SMTP Settings (2) and the email address in your Server Settings (3)? 1 - Hamburger (top right)/Account Settings - click on the name of the account through which you are sending the email. On the right is the return email address called 'Email Address' unless you have put another email address in 'Reply-to Address'. 2 - Hamburger (top right)/Account Settings/Outgoing Server (SMTP). Highlight the server through which you are sending the email and the User Name is displayed at the bottom. It can be changed using the 'Edit' button. 3 - Hamburger (top right)/Account Settings - click on the 'Server Settings' under the account through which you are sending the email. The email address is in the 'User Name' field.

Hi. Yes the user name/email address in the 'From' section of my email matches the User name in your SMTP Settings and the email address in the Server Settings.

Don't know whether this could be the problem but, unless I'm misreading something, according to the link for the config settings you provided, the outgoing server should be smtp-mail.outlook.com, not smtp.office365.com…