Mail relay for localhost?

I use a knoppix 4.0.2 DVD on my work PC when I travel (so I don't have to put up with windows). I have it running nicely with a persistent home directory on a USB key. The only thing I can't do at the moment is send mail from (e.g.) Thunderbird. I'd like to be able to run an MTA which only listens on 127.0.0.1 and will only relay clients connecting on that IP.

Can anyone offer simple instructions on how to set this up? Can I just run smail under its default configuration on the DVD or will that leave me hanging off the internet running an open relay?

If I need to alter the config, it seems I can add this to my persistent config drive. Is this the case?

Thanks for any help you can offer.

Matt.