I got the following error when trying to look at mailbox using EMS from an Exchange 2010 server. The mailbox was on an Exchange 2003 server.

WARNING: The object MY.DOMAIN/MYOUs/Firstname Lastname has been corrupted, and it’s in an inconsistent state. The following validation errors happened:
WARNING: Property expression “FLastname” isn’t valid. Valid values are: Strings that includes ‘@’, where ‘@’ cannot be the last character

It turns out that this issue was the userPrincipalName attribute for the user. It probably was missed in the migration from a Windows NT4 environment to Windows 2000/2003. I simply used ADSI Edit to add the @MY.DOMAIN to the end of current attribute setting, and all was well.