What's blocking Port 25?

[Griffin Palmer <griffjo@pacbell.net>]

This may be one for the firewalls list, but I'm not
entirely sure this is a firewall issue, or, at least,
*entirely* a firewall issue. I'm pretty sure it's not
an exim issue.

I thought I'd see if there are any troubleshooting
suggestions from this obviously very experienced user
base, before trying more specialized lists.

I've just installed testing/unstable on my machine. 

I'm running behind a NAT DSL router, with iptables on
my machine.

I have my own domain, running its own DNS, smtp host,
http host, etc. 

I'm using exim 4.41, compiled from source, as my MTA.

I was earlier running exim 3.36-11 on another box,
behind the same router, using identical port
forwarding. But in the fresh install on more robust
hardware, something has gone amiss.

I'm pretty sure it's not exim. All the diagnostics
check out for exim. I can send from and reply to
local-domain addresses, using either mutt or mail.

I can also send messages to the outside world. But
replies (or original messages) from the outside world
fall on deaf ears. Eventually, I get an "I've given up
trying" message from my ISP's MTA -- except in one
instance (more on that below). 

Here's where it gets confusing. Running nmap against
my machine from outside the domain shows it is
answering that Port 25 is closed. This even though,
using FireStarter, I've explicitly opened Port 25.

I'm *certain* I've properly forwarded Port 25 to the
correct IP address for my new machine. 

Here's another oddity: Even though my machine replies
'closed' to Port 25 scans, I got one error reply with
one message attempt from my ISP account that suggests
the port isn't *entirely* closed.

Thinking I might have DNS configuration problems, I
tried sending a message to myself@123.457.789.012 (my
user name and IP address substituted for this dummy
address). I didn't realize at the time you had to
configure exim to accept mail addressed in this
fashion.  

I got an error message back informing me that exim had
refused to forward the dotted-quad-addressed message
to my account on my host.

So it seems as if *something* is getting through on
Port 25. 

It doesn't seem, though, that it's a matter of
messages reaching my host and then exim failing to
properly forward them locally: There are no
unforwarded messages languishing on exim's input
queue.

Any troubleshooting ideas would be appreciated.

                              Griff Palmer