[exim] Setting up exim to relay through ISP's email server

I'm running Centos (technically Red Hat 4.1.2-14), and I'm trying to get
the configuration correct for using the ISP's SMTP server as the relay.
Naturally the SMTP server requires authentication. This doesn't seem to
be all that straightforward: I've tried all of a dozen different
configurations out there and most just don't work. One did get going,
but the configuration was for SMTP relay without authorization, thus the
SMTP server returned with a "client not authorized" error.

Say my isp is

mail.isp.com

and my username is

username

and my password is

password

What would my configuration be in exim.conf, with the appropriate
configurations in routers, transports, and authenticators (I'm assuming
the configuration would require something in all three sections)?

I apologize in advance if this is a simple question, but the
documentation really doesn't lend itself to figuring out a configuration
like this.

TIA.

--
Vahe Oughourlian
Software Engineer
vahe@???