Maybe OT - Relaying question

[Thomas Cameron]

All -

I have a client with two mail servers - one that a big ISP manages and
the one that will run SA 3.0.0 and spamass-milter-0.2.0+cvs-3.  My plan
is to remove the ISP's mail server from the customer's DNS zone and send
all mail to the SA + spamass-milter box, then have it relay to the ISP's
mail server.

Question is, how to I tell sendmail to do this?  If DNS points to the SA
+ spamass-milter box, then how do I tell sendmail to forward the mail to
the ISP's server?  Doesn't sendmail look to DNS to determine where to
send its messages?

I am thinking I may need to set up a bogus DNS server on the SA +
spamass-milter box to fake it into sending to the ISP's server.

Is there a better way?

Thanks!
Thomas