Multi WAN Exchange Mailserver receive error

[edwtg]

Hello

We have a PFSense setup with 2 wan interfaces, 1 DMZ and 1 Local LAN.

At the first WAN interface wa have an ADSL ISP with static IP.
This interface is used fo our mail, vpn, remote web, …

The second WAN interface has a cable ISP with static IP (with volume limitations)
This interface is used as a backup internet connection.

Load balancing is used for surfing.

Recently we discovered a problem with our incomming mail traffic.
Almost all mails come thru, except the mail that are comming from mail servers with an IP in the range of our second WAN interface.

For example (not out real ip's)
WAN1: IP: 200.100.7.65  GW: 200.100.7.1
WAN2: IP: 80.82.7.88     GW: 80.82.0.1
Incomming Mail: Server IP: 80.82.5.156

I did a packet capture on the two WAN interfaces:
The fist package of the mail we receive thru WAN1.
The problem is that the response of our mail server is send back thru the WAN2 interface.

Can someone help me to solve this problem.
I want to route all mail traffic thru our WAN1 interface.

Our Exchange sever is located in our local LAN
Only our webserver is in our DMZ

Thx

[althornin]

enable advanced outbound nat.
Set it so that all traffic from exchange server IP is routed out through WAN address:

[edwtg]

Thank for the reply.

I've added an outbound rule for the ip of the exchange server, but the problem still exists.

Is it possible that it has something to do with the load balancing?
Do i need to change something in the firewall rules?