[Hampshire] qmail not listening

Hi all,

I'm dropping out of lurk mode to tap into the knowledge of the list,
hoping that you guys can help me debug the mail set up on the server
I've got.

It's a VPS running Fedora 7 with exim. When I start it up, it says:
2008-06-01 10:42:30 exim 4.69 daemon started: pid=19977, -q1h,
listening for SMTP on port 25 (IPv6 with IPv4) port 587 (IPv6 and
IPv4) and for SMTPS on port 465 (IPv6 and IPv4)

and nmap localhost shows it's open,

which all sounds good to me, but nmap from a remote host shows port 25
closed (but 465 and 587 open).

I've checked with the tech guys hosting the server and they say
there's no reason it shouldn't be open (i.e. they're not blocking the
port), but when I swap port 25 for port 26 in exim.config, port 26 is
open remotely.

Which I think means something is blocking it on my server. I'm a
little rusty with system stuff like that, but 'iptables -L -v' shows
no active filters.

Can anybody think of something I'm missing? I'm hoping it is something
simple, but my google foo has deserted me.

TIA,
David.

P.S. wow HantsLUG's made it to 10 years old. Congrats all.