This brief guide will explain the steps you can take to get basic SMTP AUTH working with Debian Sarge's exim4 package. (For users connecting to your server, not for forwarding via your ISP)
First of all generate an Exim SSL certificate:
# /usr/share/doc/exim4-base/examples/exim-gencertNow edit /etc/exim4/exim4.conf.template using your favourite text editor.