I am going to set up my web/mail server with a second network card which will be tied to its own IP and domain name.

How do I direct sendmail to send out an email as coming from IP#2, domain #2 not IP #1, domain #1?

The only way to affect which IP address sendmail uses for outbound traffic, is to bind the sendmail process to a specific IP address. But if you do that, all outbound mail will be sent using that address.

If sendmail is bound to all addresses (the defauls setting), the routing table decides where the traffic goes, which in turn determines the source address.

I guess you could run several instances of sendmail, each bound to a specific IP address. It wouldn't look pretty, but it could work.

Why the need for a different source IP address for mail from different senders?

Thank you the reply.
The reason I'm doing this is that a while back someone hacked my server and sent tons of span as if it was me. Now my primary domain/IP is black listed among some major providers and I have to send out legitimate email for business purposes. Its fine it I have to make all outbound email tied to one IP/domain. I made the domain names similar.