multisite mail

I'm new to this group.

We've been running a multi-site hosting environment for a few years - small sites, small traffic, so easy to maintain. One problem that we haven't solved yet is how to configure sendmail on the sites to send from the domain of each sub-site. Mail comes out with appropriated "from", but Sender is listed as the www user of our mail server.

Has anyone dealt with this issue?

Thanks,

Bram.