You can identify below possible causes If Thunderbird displays an error message when you try to send mail.
1)Firewalls and antivirus software
Check if your firewall or antivirus program is not blocking Thunderbird. With a classic firewall you can set a rule that states the application is allowed to connect to the Internet. However, many firewalls/security have threat level settings. Sometimes those settings work fine with fetching mail but not sending mail. You can always manipulate the settings and allow Thunderbird Email to pass through.
Verify Thunderbird is in online mode, not offline mode: click “File -> Offline”; “Work Offline” should be unchecked. If you send a message while in offline mode, it goes to the Unsent Messages folder in Local Folders.
Your email provider may set a captcha if they think you did something suspicious. Thunderbird doesn’t know about captchas, or have any way to clear them. You can log into webmail using a browser and see if that is the case. That will give you a chance to clear the captcha if one exists.
4)Outgoing mail settings
Try to find out correct outgoing mail (SMTP) settings from your service provider i.e. from your Internet Service Provider (ISP): Try Googling for your ISP plus the word “smtp”.
Verify the settings in Thunderbird exactly match what your ISP provides: no typos (e.g., “smpt”), extra spaces, punctuation mistakes (e.g., no commas instead of periods) or capitalization errors (e.g., lowercase instead of Caps).
Click “Tools -> Account Settings -> Outgoing Server (SMTP)” (at the bottom of the left pane—you may need to scroll down), and double-check:
Above all the major problems encountered while sending email in Thunderbird.