NFS client cannot mount with iptables enabled

Issue

Mount fails when iptables service is enabled with the following rule:

# cat /etc/sysconfig/iptables

# Generated by iptables-save v1.4.7 on Tue Aug 27 13:29:46 2013
*nat
:PREROUTING ACCEPT [13:696]
:POSTROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A POSTROUTING -j MASQUERADE     <---------------------------
COMMIT
# Completed on Tue Aug 27 13:29:46 2013
# Generated by iptables-save v1.4.7 on Tue Aug 27 13:29:46 2013
*filter
:INPUT ACCEPT [28:2528]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [20:3280]
COMMIT
# Completed on Tue Aug 27 13:29:46 2013

The mount error received is:

# mount nfsserver.example.com:/export /mnt
mount.nfs: access denied by server while mounting nfsserver.example.com:/export /mnt