Traffic shaping with iptables

hello,

I have a postfix & a local dns running on a single server.
this server is connected to internet via a low bandwidth line(with fixed ip).

we also have another high speed adsl (dynamic ip).

i want to divert all dns request from the local dns & postfix
from the server to the adsl line in order to save bandwidth.

note that its very important to let the server use the low bandwidth line only because it has a fixed ip.

plz, advise how do i accomplish this.