need help getting sendmail to send from a different domain

[sectrean]

I'm very new to sendmail.

We have a dedicated server with several IPs and several domains hosted on it. We are having an issue where we are trying to send out mail from a domain on the server that has a dedicated IP. But the mail is being sent out from the main (shared) domain/IP of the server.

So we need mail from the domains with dedicated IPs to be send out from those IPs. It's fine if everything else is sent out from the main IP.

So I guess I need to set up some sort of masquerading, but I have no clue where to start.

Thanks in advance.