[Exim] SMTP data timeout - troubleshooting tips??

I've searched the Exim archives & Usenet regarding an Exim error
message when certain hosts attempt delivery to my mailserver.
For example (the namedroppers mailing list):

SMTP data timeout (message abandoned) on connection from psg.com [147.28.0.62]

The concensus is that the error is genaerally caused by a firewall
or other such device. In may case (home environment), it might be
my DSL box/firewall, a ZyXEL model Prestige 643.

I recently upgraded Exim to 4.22 with exiscan-acl patch rev 12,
tho I was also having the problem under Exim 3.36.

Can anyone supply troubleshooting tips/suggestions for determining
the exact cause of this problem, or at least narrowing the cause
to the box or software?

Regards,

- michael coxe