[exim] selective relaying

Hello,

I'm running a long time ago an exim4 smtp server. This server send all outgoing mail to a smarthost while I haven't a static IP. When I'm traveling I tray to use the smtp relay of ma guest but it is not always possible.
So, I wish to send my mail (specially answers) to my server which should the relay then to it's usual smarthost.

My question is : how to configure exim4 to tha in a safe way. I did thing I should use a form of authentication in order to ensure than only my massage are relayed. But after...

Tanks in advance for any help

Regards

Jacques Silberstein