• In the Thunderbird menu bar, click the Tools menu and select Account Settings, or click the Application menu button  and select Options and Account Settings from the sub-menu, and select Outgoing Server (SMTP) from the bottom of the list in the left panel.
  
  Check that the settings are the same as documented in the article for your mail provider (Gmail, Yahoo, Comcast, Hotmail, Sympatico…)
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Verify the correct SMTP Server is being used

• In the Thunderbird menu bar, click the Tools menu and select Account Settings, or click the Application menu button  and select Account Settings, and select the account name. Verify that you are using the right SMTP server for the email address. Most of the time, you cannot send messages from one account provider using another account provider's service. For example, you cannot send a mail from “joe@gmail.com” through Yahoo's SMTP server.
  

Verify that your outgoing email is not being blocked

Verify that your outgoing email is not being blocked by your firewall, VPN, proxy, antivirus software (which sometimes implements a proxy and intercepts email) or your Internet Service Provider:

• Try briefly turning both antivirus, VPN, proxy and firewall software off, sending a test email, and turning them on and sending another test email.

• Many ISPs block outgoing email on port 25. You may have to switch to another port (e.g. 587 or 465). Contact your ISP's support to see if they are blocking your outgoing email port.