In the create and edit aliases pages, domain owners can choose from any of the options selected here as alias destinations. Some are "special" and may introduce security problems, if your user is not reasonably trustworthy. For example, the "Addresses in file" option could be used as a spam relay, even if you have configured the server not to allow incoming email. This is only a problem in environments where you users are not reasonably trustworthy. Most users who can interact with Virtualmin should not be considered this type of user. "Delete" is also a somewhat dangerous destination as it is a rather vulgar abuse of the trust of the mail system (mail server claims to have delivered the message successfully when in fact it is simply deleted).