Double_NAT Error leads to Mail blackhole.ntp? (Airport/DSL)

Question

I'm having problems with sharing files on the homeside network AND extenal Internet and Mail.app via a shared Airport connection.
I have a PowerMacG4 (10.4.9) and older iMac (10.3.9) connecting through a Netcom NB5 DSL/modem/router - on their own NO problem for ISP mail, gmail, internet.
But then the problems start when I trying to connect my MBP via airport.
So I set up the PowerMacG4 to share its Internect connection to computers connecting via airport with WEP.
Downstairs, I turn on Airport on my MBP. {Sometimes it takes many failed attempts to connect}
Okay so MBP gets connected and in Network Preferences/Airport is green for go - connected to internet via "G4Air13".
At this time, the Finder/Network shows three aliases created by the system - Libary, Servers, PowerMacG4. If I try to double-click the presumed PowerMacG4 'server' then the beachball spins for 120 seconds (while other applications may be more responsive).
So No go with AFP.
Okay so I set up MacFusion, and with a bit of fiddling and eventually JOY I succeeded in mounting the PowerMacG4 on my MBP downstairs via SSH - comes up in an instant as a 'volume' on the desktop. Transfer copy delete files all okay, launch app over network okay too.
So MacFusion/sshfs solves the file transfer issue, rather than AFP ...
At this point, Safari delivers external sites or whatever - but Mail.app won't send or receive anything from Yahoo.
Looking at the netstat - a various times during testing I noticed that Terminal was listing the destination ip of one of my attempts to be "blackhole." and further down a tcp connection was listed as "blackhole.ntp". This doesn't seem correct.
I've found a few recent theories hereabouts like -
1) ISP's are blocking/filtering PORT 25
2) Airport is unstable.
2) Mail.app or SMTP authentication is breaking.
My theory is that something is going wrong with the routing and firewalling between Airport acting as router and Netcomm DSL acting as router.
For example the IP's for the setup described above is :
NetCommDSL Router 192.168.1.1 (static itself, DCHP range 192.168.1.100-192.168.1.199) NAT on Firewall OFF
PowerMacG4 Ethernet DCHP-auto 192.168.1.100 (submask 255.255.255.0, router 192.168.1.1)(share ethernet to airport)
PowerMacG4 Airport DHCP ??.??.??.?? (submask blank, router blank)
iMac Ethernet DCHP-auto 192.168.1.101 (submask 255.255.255.0, router 192.168.1.1)
MBP Airport Manual address 10.0.2.5 (submask 255.255.255.0, Router 10.0.2.1)
So then - MBP sends request via Airport (10.0.2.5) to PowerMacG4-AirportCard-acting-a-NAT-Router (10.0.2.1) shared to PowerMacG4-Ethernet (192.168.1.100) to pass it on to NetcommDSL-as-NAT-Router (192.168.1.1).
MBP can traceroute each of these IP addresses above.
One hop to 10.0.2.1
one Hop to 10.0.2.5
One Hop to 192.168.1.100
Two Hops to 192.168.1.101 (via 10.0.2.1)
Two Hops to 192.168.1.1 (via 10.0.2.1)
So have I got a double-NAT problem or does or this look okay?
MBP Net Diagnostics all green for ISP/internet.
MBP No connection doctor result
MBP Still no Mail ...!
Thanx
:g

Answer 1

I guess I run WaterRoof on the PowerMacG4...
Yes. It is a tool for configuring the firewall.
I guess I should be trying to allow mail traffic through the PowerMacG4 via port-out 25 and port-in 995 rather than forwarding-ports...?
You need to allow port traffic on those ports for the AirPort interface. The firewall has no effect on out-going traffic.
I guess I should still aim to have (appropriately configured) firewall running on BOTH the PowerMacG4 AND the MBP via airport...?
There are no changes required to the firewall on the MBP.
Or do I need just one firewall at the PowerMacG4...?
The firewall on the MBP would be a deterrent against wireless attacks.