BleepingComputer.com: Can't connect to internet via proxy

Hi. I have a linux(fedora) which i use as a proxy. the proxy then connect to the router. the proxy also serves as my mail server (i use postfix). Problem is, since a few days ago i discovered that i cannot access the internet for any machine connected via the proxy (and any dhcp-assigned machines since the proxy also handles dhcp).

this server being a mail server, even the webmail is not accessible from any machine.

When i tried, i changed some computers so that they connect directly to the router by setting default gateway to be the router -and they work fine for the internet. Problem is, I am still not able to access webmail.

What could be the problem with the proxy.?(NB Postfix is running and http too is running).
Kindly help
rgds