Why am I unable to add an email alias?
If you’re having problems adding an email alias, there are two possible reasons:
The alias is already attached to another active mailbox
Check the list of existing addresses for each mailbox on your account to see if the alias is already in use.
If it is and you wish to transfer it on to another mailbox, you need to delete the alias from the current mailbox. For further information on how to do this, please see How do I remove an email alias?