[Exim] ssh tunnel to remote's 127.0.0.1:25

A> You can write your email offline, and then make a ssh tunnel with your
A> ppp connection.
A> eg: ssh -A jidanni@??? -L 2500:127.0.0.1:25
A> And then configure your nullmail as "localhost smtp --port=2500"

http://www.hserus.net/exim.html does tell you that ...

Is it something like in /etc/ppp/ip-up.d/mailjazz I do
ssh -A jidanni@??? -L 2500:127.0.0.1:25
and in exim,conf:
route_list="*.tw 127.0.0.1:2500 byname; *.nurdco.com ...; ...;" ?
The spec says the : is a separator though.