by default I set the nat chains (PREROUTING POSTROUTING, etc) to ACCEPT. first off is this bad?

For my web & mail servers using DNAT, I need to add rules - and I do. Same w/ SNAT rules...

The problem has come up with our mail server - we were getting spam from one IP address - since my DNAT rule was ACCEPT and forward, the mail server kept getting these spam messages. I stopped the SPAM by adding a rule to the beginning of PREROUTING to DROP packets from that address.

This made me start thinking - am I doing it right by having all the nat chains default ACCEPT?

aldo