Sendmail: Proxy server or whatever

[Joel Hammer]

I am on comcast cable. I run sendmail to directly send mail to my
recipients.

Of late, some sites, eg. aol.com,  are rejecting my mail, telling me I
should be using my isp's mail server. 

Comcast can be a very linux hostile environment. I don't really want to talk
to them about mail. However, I would like to either relay through their mail
server or masquerade my mail to have their mail server's ip.

I have the Bat book on email, and am not afraid to use it, but, I am
confused a bit.

Any pointers, specifically clear examples, on how to do this would be
greatly appreciated.
Joel