What OS and version of avast! are you using? Is this an encrypted mail system like gmail or non SSL? Was email working previously with avast! or is this a new installation? What is your TB server setup?
Mozilla says this error means:
An error occurred sending mail: The mail server sent an incorrect greeting: message:
You specified the wrong server. The server you specified exists, but it is not an SMTP server.
You specified the wrong port number. Ask whoever runs the SMTP server what the correct port number is.
The server is down. This is usually temporary. If it persists, contact whoever administers the server.
Your firewall is blocking the port.
Your ISP is blocking the port. This often affects port 25, and you can often work around it by using port 587, but details depend on your ISP and on the SMTP server's configuration.