The problem depends on your network model...
External mailing list, expanded internaly on the mail hub. But the
problem is if the Web hosting machine is not the mailing list machine
you have problems.
Namely DB sync problems. Because either you are trying to keep a link
between two machines up, which may or may not be a problem (ODBM type
model) Or an rdist type cron driven link.
Models and problems-
Web Host/Mail Host, same box. Once you get the privs worked out right
it should just work.
Web Host and Mail Host Different boxes, external net, but close to
each other. It shouldn't be too bad to keep them in contact with each
other, you could even do something like NFS mount a lock directory and
the mailing list.
Web Host and Mail host differnet boxes, external nets, differnet
sites. Say you pay someone to do your web hosting but your mail
hosting is local to your site. Now you have to maintain connectivity
between those two sites some how. And if the link goes down you have
to do something queue up the requests. Or not take requests or
Web host external, mail hub internal. Now what do you propose to do?
Open a custom channel between the hosts?
I think baseing everything on the web is a mistake. The mail channel
to do everything is still required.