If you cannot send messages, this page provides a series of steps you can use to check for common problems.
Verify that your outgoing email is not being blocked by your firewall, antivirus software or your Internet Service Provider:
Try changing your SMTP password. This is usually done via a 'reset email password' form on your ISP's support website.
Please create a new support request with a screenshot of your SMTP settings with your userid obscured and include the following information: your mail provider (for example, Gmail), ISP (for example, Comcast), firewall version (if any), antivirus software and version (if any), operating system and version (for example Windows 7 or Mac OS X Mavericks), Thunderbird version (for example Thunderbird 38.2.0). See the How do I create a screenshot of my problem? article for information on how to create a screenshot.