The obvious thing to check first is the outgoing server settings.

Under 'Account Settings' click 'Outgoing Server (SMTP)' in the left hand column. Double click the 'Outlook.com (Microsoft)' entry and check that the details are:—

 Server Name: smtp.office365.com
 Port: 587
 Connection security: STARTTLS
 Authentication method:  OAuth2
 User Name: <your e-mail address>

  That's the obvious thing. If all is as it should be, and you're using Windows, it could be anti-virus software causing trouble.

What happens when you try to send a message? Does it appear to work but not? Does it produce an error message?

Hi FredP

I know in the past you had smtp outlook setting smtp-mail.outlook.com But as you know microsoft have recently changed a lot of things. That includes server names and most importantly the Authentication Method.

So , are you using the most up to date settings ? To answer this please post the following information. In Thunderbird

• Help > Troubleshooting Information
• Scroll down to 'Mail and News Accounts' section
• Copy all the data
• Paste the data into this forum question.

Have you set up 2 step authentication and created an app specific password ? OR are you using Authentication Method: Oauth which means you do not use 2 step authentication.

You say using the latest version , but there have been several updates recently. Are you using 128.2.3esr ? if no then update to that version.