Pfsense Multiwan with DNS

[hwuoi]

I had successfully setup a multiwan load balance and failed over. However i cannot received from external, but i still can sending mail out.
And i suspect due to te DNS problem. Any one can helping me on this?

My network diagram

LAN –--> Proxy Server (also DNS Server/ mail server )---->Pfsense Firewall  -----> WAN - public IP
                                                                                                        -----> WAN2 -Public IP

cos all other is work perfect like browse internet and etc. but not received mail from outer world, like yahoo, hotmail and etc..
Any help will much appreciated. Thanks