ipmasq/ipchains and ping

I have, what seems to be, an odd thing happening with my home network
gateway.

I use a high-speed dialup to the web and had set up my firewall with ip
masquerading. On my workstations I can access the web, dns servers and most
of my software that uses the net, but I cannot ping anything outside of my
local network. Ping shows me the correct IP address for the domain name I
give it but then no packets are transferred. If I give it the IP address no
packets are transferred. Even from the gateway machine I cannot ping
machines on the web. This , I believe, causes a problem with my mail
software where it cannot sent email out. I am assuming it is trying to use
the same mechanism as ping to verify the existence of the receiving domain.
I believe that the problem lies in the way I have set up the ipchains,
although, I am not sure what I am missing. I went through the HOW-TO and
tried to implement that but then fewer things worked than before. (As you
can probably tell I am not an ipchains guru by any stretch of the
imagination)

It had been working some time ago but then stopped at some point. Not sure
when. I have also heard that some isp's can, and do, filter this type of
network traffic out in some cases? Not sure if this is true or even what to
ask them to see if they are or are not.

Any help would be appreciated.

Chris