If you mistakenly create a mailbox user with ADUC ( Active Directory Users and Computer ) you will notice that the mailbox you created for that user is a tagged as a legacy mailbox. You will loose some features of Exchange 2007 in this mode. To fix this you need to remove the legacy tag in Exchange 2007. The following cmdlet will make it a User Mailbox when you run it in Exchange Shell.
Set-Mailbox -Identity "alias" -ApplyMandatoryProperties
Just change "alias" with the mailbox name or the alias and keep the quotations on it.