I'm wondering what is the relationship between virtual hosting of a web site domain vs. a
mailbox domain

I have had an account on impulsedata.com, which is an ISP in town here.  They host a few
domains of mine, all of which are mapped to the same place (one web site, lots of
aliases).  All of the email gets mapped to my main userid at impulsedata.com.

I've played a bit with a non-dedicated system hosted on a Cobalt RaQ3 and RaQ4.  I can
route a domain by changing the DNS entries at DirectNIC or InterNIC, and the host needs to
know what the domain name is so they can set things up there.  This works for single
domains.  Neither host would support aliases of either web or email domains.

Some hosts advertise that they'll allow virtual hosting of multiple domains on one site
(like what I'm doing at ImpulseData.com right now), but they they say they won't allow
multiple email domains.  IOW, I can set up a main virtual host as abc.com, then aliases of
abc.net, def.com, and def.net, but they won't allow email to the latter three to be
delivered.

I know that you can flip things around from Apache's conf's, like fred.abc.com vs.
www.abc.com/~fred.  But how are these kinds of things handled for email?  I get the
impression email routing is handled differently, although the domain naming seems like it
ought to be subordinate to the web routing.

Thanks
-David