Not Valid for UserPrincipalName Error when Moving Mailboxes with Exchange 2007

While moving mailboxes with Exchange 2007, you may encounter an error message such as:

"username" is not valid for UserPrincipalName. Valid values are: strings that include @. where @ cannot be the last character. It was runningcommand move-mailbox - TargetDatabase Exchangeservername\First Storage Group\Mailbox Database.
Open mailbox properties in Exchange Management console for the mailbox encountering the error mesage. Recipient Configuration - Mailbox. Right click mailbox displayed in error message, click properties, click the Account tab. Select a domain name from the drop down box nect to the User Logon Name field.

Mailbox will now move successfully without error message.