If an account is created by an incoming email (i.e. automatically), it appears that you cannot delete the login username.
i.e. we have an account that has been set up by a reporting programme reports@domain.com and the login is reports@domain.com. As this account does not need access to the self support portal on our system, I want to delete the login name but leave the account. It seems that SD+ will not allow me to do this at present.
In the meantime, I will delete the account and recreate manually without putting in a self support login.