ENDIAN Firewall - SMTP conversations timing out

I have a strange problem that I can't figure out how to debug.

I have a Qwest DSL line with static IPs using an Actiontec PK 5000 modem/router. When configured with a block (/29) of static IPs,
the router acts like a bridge.

I started with a SBS2008 Server running Exchange 2007 using a smarthost to send and receiving email directly for my domain. About
30% of the incoming email would timeout after the remote server sent "DATA" and my server responded with
"End data with <cr><lf>.<cr><lf>". After 1 minute (later modified to 2 minutes), my server would close the connection. There
are no problems with outgoing email.

I then changed the configuration to use an Endian firewall with the SMTP proxy and this is working except that now 30% of
the incoming email is timing out coming into the Endian SMTP proxy. I want to keep the Endian solution as having my servers directly
on the internet worries me.

The are the same on both configurations so I'm guessing the problem lies somewhere upstream. How can I troubleshoot
this problem? The only thing I've come up with is to look at the SMTP conversation at both ends, but that requires access to a properly
configured SMTP server (and its administrator). I can't find one of these.

Let me know if you have any ideas on how I should proceed. I'm located in Colorado Springs, Colorado, USA.

Regards,

Brad