Adds a new user to the mail system with userName. For POP3 style stores
this may only involve adding the user to the UsersStore.
param
sender - the name of the user
return
a reference to an initialised mailbox
deprecated
addUser should not be considered a property of a MailServer
We could have readonly userbases providing full MailServer implementations.
Look at the UsersRepository.addUser(username, password) method.