Masquerading help

I have a couple of sendmail based SMTP gateways that, among other things,
are now relaying email for a variety of new servers which send the email as
user@hostname.domainname where the domain name is a private not legal domain
name. I've been trying to get the appropriate sysadmins to change that, but
in the meantime, I was thinking I might be able to something on the SMTP
gateways with masquerading.
But I'm not sure how to keep the various hostnames in the address (and I
don't have access to my Bat book - I'm working from home for awhile because
of health problems)

Thanks
eric