viaaliasrequired specifies that any final recipient address that matches the channel must be produced by an alias. A final recipient address refers to the match after alias expansion (if relevant) has been performed. The address cannot be handed directly to the MTA as a recipient address; that is, it is not sufficient for an address to merely rewrite to the channel. After rewriting to the channel, an address must also expand through an alias to be considered to have truly matched the channel.
The viaaliasrequired keyword may be used, for example, on the local channel to prevent delivery to arbitrary accounts (such as arbitrary native Berkeley mailboxes on a UNIX system).
The default is viaaliasoptional, which means that the final recipient addresses that match the channel are not required to be produced by an alias.