[Exim] host_accept_relay

I've been trying to send mail to my new mail server and I can't seem to
make it work.

There could be any number of problems, since I'm new at this, but so far
I've set up Exim and Courier IMAP and when I test locally everything
works. I've installed Squirrelmail and that works, at least for sending.

But no matter what I try, I cannot send mail to this server.

I have to assume that our ISP has blocked mail or I've set up the
host_accept_relay incorrectly.

Any suggestions?

Thanks, Brian