load balancing inet-connections

I want to be able to send and receive mail through one internetconnection
and to surf the web loadbalanced through 2 internetconnections.

My linux box is connected to 2 isp's using iproute2 and load balancing. It
has postfix and netfilter running, and acts as a NAT gateway for the
internal network.

MX record for our domain points to the ip address of only one internet link.
I want to keep it his way.

Users on the internal network have no problems surfing the web, however, our
mailserver, running on the linux box, cannot connect to other mailservers on
the net.

Marking packets destined for port 25 in the OUTPUT chain and adding a rule
with fwmark pointing to a table with a default route via one ISP does not
help.

When i disable the second link and adjust the routing tables accordingly,
the mailserver is able connect to other mailservers.

Could someone help me,

I AM DESPERATE!!