[Exim] defining ip-adress of local interface

Hello,

I configured exim to listen to one port on one fixed ip-address only.
But my problem is, that on the same server, there are some virtual
ethernet interfaces, so exim doesn't send with the ip-address of the
local interface only. How can I force exim to use a certain ip-address
when sending mails through smtp to other mailservers?

Regards
Marten Lehmann