Meeting Request Responses From Alternate Email Address

We have a client who we've corresponded with meeting requests for quite some time. Recently, this no longer works. The meeting requests/updates evenually timeout. I found that when this particular client responds to meeting requests, they come from a different address than where they were originally sent. Example: the email address is username@domain.com which is a valid address and one used for email correspondence on a regular basis. When they respond to the requests, their responses come from username@one.domain.com. The username@one.domain.com isn't a valid domain for receiving email so if the there are any updates or cancellations to these meetings the updates aren't delivered since the name doesn't exist and they eventually timeout. Has anyone experienced anything similar? I realize external meeting requests aren't ideal but it's a large client so I need to exhaust all the possibliities before throwing in the towel. Is it possible to make a rule on Exchange that would allow me to automatically change the address from username@one.domian.com to username@ domain.com? I'm using the Exchange 2003 with all the available updates and service packs (soon to be upgraded to 2010).

Thanks in advance! - Dan