How to properly setup return path on sendmail?

Question

When I use the function mail() in PHP my e-mail comes out of my server with the following line:

Received-SPF: neutral (google.com: xxx.xx.xx.xxx is neither permitted nor denied by best guess record for domain of [email protected]) client-ip=xxx.xx.xx.xxx;

I just replaced my server name and IP to: xxx.xx.xx.xx

I host quite a few websites, so how do I get it to set the return path without hardcoding it using -f option on sendmail. Is there a easier way to setup a SMTP server?

Thanks,