I came across this error today while getting the properties of a legacy mailbox in a co-existing Exchange environment (2003 & 2010). The same error will be thrown if you try to move the mailbox from Exchange 2003 to 2010.
WARNING: The object domain.local/Users/User has been corrupted, and it’s in an inconsistent state. The following validation errors happened:
WARNING: Property expression "User" isn’t valid. Valid values are: Strings that includes ‘@’, where ‘@’ cannot be the last character.
I immediately checked to see whether there is an “@” at the end of the username, but there wasn’t!
The issue was that the UPN was not filled in properly. The “domain part” was blank and Exchange sees the account as “username@”, which it doesn’t like.
Once the UPN was filled in properly, Exchange was happy and I could move the mailbox to 2010