Steps to Fix Unable Send Email Messages in Thunderbird :

If you are unable to send email messages from your Thunderbird account, there can be two reasons behind this:

• Inappropriate Thunderbird Settings
• Blocked Port 25 by ISP (Internet Service Provider)

Solution – To resolve this error in Mozilla Thunderbird, perform the following:

Step 1: Ensure that Thunderbird settings are appropriately configured:

• Incoming mail server (POP3/IMAP server): mail.yourdomain.com
• Outgoing mail server (SMTP server): mail.yourdomain.com
• Username: the full e-mail address like [email protected]
• SMTP authentication must be enabled for the login to be successful.

Step 2: Try using alternative TCP port 2525 for SMTP Server.