Problem with Domain name structure on Exchange Server 2003 on Windows 2003

Our domain is x.corp. Exchange server in domain is mail.x.corp. Some email servers are not accepting our emails because they expect to see recieved from instead of mail.x.corp when they get email from
How many ways are there to fix this and which is the easiest? For example instead of trying to change the domain name which I consider not easy, I thought maybe I can add a record in DNS where mail.x.corp and are the same.
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James MontgomeryCommented:
Are you referring to the mail banner on your smtp server or the reverse dns lookup that some people use as an anti spam measure?

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