[exim] Smarthost through ssh-tunnel

Hello,

I would like to tunnel a special email-address through ssh, because it's
relayserver is firewalled. I want exim to trigger a tunnel when
smarthost is required (not very efficient, but the address is not even
used one time a day, so a permanent tunnel would be overkill).
My question is: how do I tell the router or the transport for this
specific smarthost to execute ssh before sending the mail?

Thanks for any answer,
Dirk