Why can't I create abuse@ or postmaster@?
The SiteHost Control Panel will not let you create accounts with these aliases as they are reserved by the system.
Why are they reserved?
In the email world these aliases have special significance and are required to meet the mail RFCs (standards).
The RFCs require that email domains have these two aliases defined, postmaster@[xxx.xxx.xxx.xxx] and abuse@[xxx.xxx.xxx.xxx]. Anyone should be able to send email to these addresses and expect that it will be delivered.
For example from RFC 2821:
Any system that includes an SMTP server supporting mail relaying or delivery MUST support the reserved mailbox "postmaster" as a case-insensitive local name.
One of the most common uses for these aliases is that they are typically required as part of the process of removing your email domain from a blacklist which has flagged your address as sending spam.