login via mailaddress and/or mailalias
currently it's possible to login to mailaccount with mailaddress and optionaly login with mailalias (login name). It would be fine to choose at each mailaddount if user can login via mailalias only (checkbox).
People tend to use one password for many accounts, so if password is leaked all other accounts are endangered. For "old style" security reasons (old style two factor authentication) it would be smart to choose if only alias is allowed to login, as alias usually isn't known to hackers.