sendmail header rewrite hack

a customer wants us (via an opportunity they have offered my fax2you business) to hack my mail server so that mail sent to an e-mail address at will cause sendmail to rewrite the header so the mail actually gets forwarded to the RHS of the virtusertable or aliases entry for that e-mail address and the From address gets rewritten as the original To address.

basically, we're receiving e-mail destined for a clients fax machine via a service at but maxemail authenticates the incoming e-mail to fax request based on From address and BP wants the fax recipient to pay for the fax. so, the From needs to be the recipient of the e-mail. the RHS of the virtusertable/aliases entry will actually be something like 13178330278 at so the service knows where to send the fax.

got any ideas?