We had 2 email filters (s1 and s2) which used to scan inbound mail and deliver it to another server (s3), outbound emails were scanned and then sent out, to s3 if the address was local, or to the internet.

Recently one of the filters (s2) crashed. I have successfully been able to configure postfix and it can handle outbound email in s2. However, this s2 is behind a NAT. And inbound emails are not coming through s2 to s3. They inbound mails are actually getting bounced. Is there any setting necessary for postfix server running behind NAT?

Post the /var/log errors that you are getting.