respond on the interface request came in
 

i have 2 internet connections, but can only utilize one at the time
the problem is i need to somehow set up that when a connection request comes on eth1 the response is sent to eth1 not eth0 where the default route is set to
on eth1 i have a slow but permanent line with static ip, which my mx record is set to, on eth0 i have a broadband cable line, ip is assigned with dhcp
what i need is to browse, ftp, ... from my lan (eth2) through the cable and everything that requires static ip (mail, remote ssh connections) should go through the second line - maybe when sending mails route it through the broadband line too?
i tried to do it with iptables/iproute2 but without success - when somebody was sending mail to my destination, the request came in, but response was send through the other interface (default route), with different ip, so the connection could not be established, the same when i tried to connect using ssh
is this possible to achieve or not? if so what do i need to do?