I think that Keef is correct.
The following FAQ page gives very similar advice:
Multiple SMTP servers (Thunderbird)
You can set up Thunderbird to use more than one SMTP (outgoing) server, or to use the same server with different usernames and passwords, so that each of your mail accounts can have its own SMTP server configuration.
If you follow the instructions to add an SMTP server the configuration that you need appears to be supported (in Thunderbird 1.5 at least) - Specify a username and password but set "Use secure connection" to "No".
Regards,
RC