Thunderbird is a popular email program created by Mozilla who also make the Firefox web browser. It's free to use and has a similar interface to other email programs such as Microsoft Outlook. It also has to be configured with the incoming and outgoing server information from your Internet provider to be able to send and receive email. If you don't have your server settings configured correctly then you will not be able to send and receive email and will see errors in the program.
When trying to send mail using Mozilla Thunderbird you may get a message that says the following:
An error occurred sending mail: Unable to authenticate to SMTP server smtp.yourdomain.com. The server does not support any compatible secure authentication mechanism but you have chosen secure authentication. Try switching off secure authentication or contact your service provider.
This error can be caused if you have your SMTP (Simple Mail Transfer Protocol) server settings set to use secure authentication when your mail server does use secure authentication. Your ISP may or may not use secure authentication and there is no real way of knowing unless you contact them to find out or just change the setting and see if it fixes the problem. It is a simple process to change this setting and get your mail working again. To do so, click on Tools, Account Settings and then click on Outgoing Server (SMTP) on the bottom left of the window. Next click on your outgoing server in the list and click on Edit.
Finally uncheck the box that says Use secure authentication and click Ok then test Thunderbird by trying to send email again.