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 
AWESOME! Genius
Thanks Lou. Glad that the article helped.
Wow! I was looking for a solution for this and accidentally came across this site!
It is solved the issue for me (Had to do Start-ADSyncSyncCycle -PolicyType Initial)
Thank you so much!
Glad to help Oded.
This is exactly when Google pays off. Quick, easy solutions like this! Excellent post, Raj.
Thanks Andrew.
So easy! Thanks for posting this. Solved my issue here in 2018. :)
Glad it helped Kenneth
I had a similar issue to Philip, the UPN domain part was not blank in the Exchange Management Console, but it was blank in the AD user. Once I changed it there I was able to disable the mailbox
Thanks Blair.
Thanks Rajith! Perfect!
Glad it helped you Bruce.
Thanks Rajith, this was a big help.
Glad it helped you Rick.
I have managed to fix this, I had to move the user to the Right OU in AD as this had been moved.
Thanks for the update Phillip. I am sure it will help someone.
I have the same issue but my domain is not blank
I am on Exchange 2013, This is only affecting one mail box and user account. I can still access the account on a users PC but i now need to give access to a new user. This error is stopping me from adding this to the new account.
Full error: Property Extension “Username@” isn’t valid. Valid values are: Strings that includes “@”, where “@” cannot be the last character.
I have checked the username and this isn’t the case.
Thanks
Phillip
Hi Phillip,
I have come across this couple of times, but the fix mentioned in the article always worked.
Did you figure out your issue at the end?
You Rock Man, Right on spot…
Thanks Farhan.
I can confirm this works on Exchange 2013. We had accidentally deleted some AD users accidentally using Delete instead of Disable in Exchange and recovered them from AD tombstone. That UPN field had not populated back on those users.
Much appreciated!
Thanks for the update Jeremy.
this worked a treat!!! not sure how this happened, but suspecting our IDM account provisioning tool.
Thanks Rubspan
Thanks, this helped.
Glad to help Gaurav.
Spot on! Thanks for the post. :)
Now to figure out how these fields got blanked out to begin with, as these were populated before……..
Glad to help Allan