It's come up before that "aliased" hosteddomains sometimes don't do what users expect them to. The man page for "makehosteddomains" indicates that if "mail.domain.com" is aliased to "domain.com", then an address of the form <addr...@mail.domain.com> will be rewritten as <addr...@domain.com> and delivered to the local mailbox for that name. The consequence seems to be that if <addr...@domain.com> is an alias, delivery fails.