Setting Up an SMTP-Relay on System with ispcp

[Mysterious]

Hi,

i would like to configure postfix to accept mail from my external mailserver which will acting als smart host and my ispcp-server as an mail-relay. Could someone describe me how i can securely set up an smtp-Relay for my external mailservers which has an dynamical assigned ip. Would say i have to do an authentication via Certificate or encryptes username an passwords. Would be nice getting an example how to do this.

Thanks

Mysterious