OK, I found a work around for any one who might be interested (this worked for me but might not for you) Go into SBS then the user you want to modify, first change the users alias email to anything you like, apply, then change the details of the user that you wanted change like mailbox quota for instance, again apply. Now if you try to change the email alias back to the original it will throw the error as before so you have to exit SBS console with the changed email alias in place then go into Exchange itself and then find the user that you previously modified in SBS console there you will see the modified email address, simply change it back to the original and apply ~ Done.