My ISP runs two SMTP mail servers, one open, the other
not; for a very
reason. A large number of customers travel across
Europe, and it is not
practical for them to always dial the ISP at international rates. Hence,
there are a number of agreements between several ISPs in several European
countries, which allow customers registered with one ISP to use the
points-of-presence of another. However, the roaming user typically wants
to keep his normal email address (ie, have mail appear to originate from
his normal address). To do so, he must either use his "home" server
despite his temporary IP address/hostname not matching that server's
(which would look like an attempted forgery to a
spam-blocking server) or
must use the "local" server and forge the sender address (which would also
fail on any normal spam-blocking server).
Open relays have NOTHING to do with the above situation - talk about
throwing the baby out with the bathwater - the above scenario is akin to
saying "gee, I want to allow http through my firewall, so lets just open the
firewall up for all ports and all services to and from anywhere". Sheesh.
There are two other much more "standard" methods of allowing the above.
First, the user could still check his email at his original/local provider
via pop or imap, and he could send mail from the smtp server at his
nonlocal/traveling provider. Second, any time an ISP sets up an agreement
with another ISP to let their users roam, they could just add the roaming
ISP's domain name to their sendmail.cw file to allow relaying from that
domain only. The easy (and recommended) approach is to do BOTH of the
above - one for inbound and one for outbound.
On a properly configured pop and smtp server, neither of the above methods
appear as forgeries, and neither method is blocked by any spam-blocking
methods I've ever seen.
Both of these situations are very easily handled by any modern mail client
which lets you have more than one mail account configuration and for each
account lets you specify inbound vs. outbound servers. Yes, this type of
setup would cause really antique mail clients problems, but the hardware
those old mail clients run on isn't exactly something portable you'd be
carrying around the country <like a PDP - GRIN>.
I might add also that neither of the above scenarious are problematic with
MAPS either. Anyone have any other scenarious where a completely open relay
is a <shudder> good idea? :)