OT: Sendmail store/forward Server

[greyhair]

Hi.  Sorry for this off topic question but I don't know exactly how to
do it an am unable to experiment at the current time.
in sendmail how do i setup a store/forward server in addition to on
going email server.  Is it domain routing?  I want a way to be someone
else's secondary mail server and my own primary mail server (and vis versa).

I know the DNS part ... just not the sendmail part.  any help or
pointers will be greatly appreciated

thanks
greyhair