There is a single ".redirect" file for your site's email addresses. All email is redirected according to the rules in that file, and only the first rule for a given name will be used.
If you have domain "a.com" and a parked domain "b.com", the "info" entry in your ".redirect" file will be applied to both "email@example.com" and "firstname.lastname@example.org".
If you park multiple domains on your site, email addresses must be unique across those domains if you intend to redirect them individually.
Our parked domains are primarily intended to be used for mirrors (alternate names for your main site). If you choose to try to save money by cramming multiple domains on one site, this is one of the limitations you must endure. Our prices are so low you might want to just get separate sites for each domain.
There is also an extensive online manual available.
http://hostings.com 888-404-HOST (toll-free US/PR/VI/GU), 706-897-4743