prerouting and postrouting + clamav + clamsmtpd

My network design looks like this:

WIN1
WIN2 - GW1 - MAIL SERVER - GW2
WIN3

GW1 iptable rules:
PREROUTING
REDIRECT tcp -- eth0:1/24 MAIL_SERVER tcp dpt:smtp redir ports 10025
POSTROUTING
SNAT all -- eth0:1/24 anywhere to:eth0

WIN are on the internal network, GW1 does SNAT. I've had a lot of virii problems on the internal network but MAIL SERVER logs show the GW1 ip. I installed clamav+clamsmtpd and added the PREROUTING rule. The problem is I don't get connections on localhost:10025. I can't access the internal network (WIN) to telnet and check why. Does the current firewall configuration have any flaw? I added the PREROUTING rule to pass any traffic to the MAIL_SERVER through clamsmtpd for scanning.