I'm not certain if I can give you a "good" reply, but rather an understanding of what I have done!
As you are aware, with PN, you have a default mailbox (to which EVERYTHING goes to, UNLESS you have instructed otherwise)!
You can then set up additional mailboxes (via the portal) of the form
[email protected]. These would be used by additional users, each using (if necessary) their own Email Clients.
You can also set up re-directs (but NOT from the additional mailboxes) of other email addresses (again of the form
[email protected]) - this can also be done for "default" names such as
[email protected].
Providing every email address that you use (or orhers use) is covered by the additional mailboxes or by re-directs, then the original default mailbox becomes redundant.
Once you have established that your default mailbox is NOT/NEVER used (it is always worth checking & double-checking first), then you can request PN to "blackhole" (basically auto-delete) anything that goes into this mailbox (NB none of these are recoverable). It is this action that reduces the risk of receiving "bounces" from non-deliverable spam messages where the spammer has used your domain.
With respect to aliases, I'm assuming this is where you have your own domain registered & are using the PN mail-servers. These are handled in the same way as the normal PN accounts. In other words, fred @mydomain.co.uk is handled the SAME way as
[email protected].
Apologies if my explanation is not particularly clear, but I hope it helps!!