Are thye exchange server users
Does user a have user b's address set for text only?

Signature
Diane Poremsky [MVP - Outlook]
Author, Teach Yourself Outlook 2003 in 24 Hours
Coauthor, OneNote 2003 for Windows (Visual QuickStart Guide)
Author, Google and Other Search Engines (Visual QuickStart Guide)
Outlook Tips: http://www.outlook-tips.net/
Outlook & Exchange Solutions Center: http://www.slipstick.com
Join OneNote Tips mailing list: http://www.onenote-tips.net/
>A user receives an invitation from a person as an email , but when she
> receives it from other users it works fine.