Server :: Need Smtp Server To Show Each Domain Name In The Headers?

I have a server that runs on a static IP address with many hosting accounts. I am using the IspCP to manage the different sites. When a mail is sent, the headers include the box's own domain name: [URL]. Now [URL] is no longer active, so if an email from another account: [URL] sends mail, there is possibility that this mail is rejected by the receiver, if the receiver system does rDNS.

The receiving server will check the headers smtp sent from [URL] and come up as a fake, because [URL] is no longer registered. I wonder if there is a way to make the smtp headers reflect only the domain name that sent the mail. Then if joe sends mail, the server should replace[URL] with [URL]. If this is possible, then users that send mail will not get bounced because the rDNS lookup will map [URL] to the ip and the mail will be accepted.