In order to send mails over an external SMTP mailserver, using STARTTLS encrypted authentication, you have to add the credentials to the following file:
The password is stored in plaintext, so make sure this file is only readable by root (and "Debian-exim" user). This should be the case anyways by default.
Jumping out of an airplane is not a basic instinct. Neither is breathing underwater. But put the two together and you're traveling through space!