[exim] How do I authenicate with my ISP?

My ISP, Bellsouth, requires a user/pw when acting as a mail relay for
obvious reasons. Thunderbird and all the other GUI mailiers have a section
in the SMTP for authentication. However I'd like to use the mail command
line program also, but I'm having trouble figuring out how to send the user
name and password. From the DOCS and the FAQ I see references to
authentication but that appears to be using exim when it is serving clients,
not when it's acting as a relay to another server.

Thanks,
Jim.