Exchange Server and Reverse Lookup

We are having a problem with reverse lookups on our exchange server. As background (names changed to protect the innocent), our internal name of our exchange 5.5 server is "exchsvr.corp.x.com" (x is substituted for company name). We have a PTR record pointing our mail server's public IP address to mail1.x.com. Everything works fine unless another company requires reverse lookups. In this case, the email message shows as being sent by exchsvr, but the reverse lookup on the IP comes back as mail1. When that happens, the other company treats our mail like spam. We want to protect our internal naming structure from being public knowledge. Is there a way for Exchange to specify in its header that mail is coming from mail1 as opposed to exchsvr? I am sure you can do it, I just don't know how. Thanks!