There are two things that I have found that should be changed to use the external relay: Mail.app and Postfix. Mail.app, the built-in mail reader, can be configured through its preferences GUI. I don't remember all the individual steps, but I remember it being straightforward. The first time you send email, you'll be warned about an untrusted certificate. In that dialog, you can specify both that the certificate is good for sending email through, and that the certificate is in general legitimate.
The other thing that might need modifying, depending on what you do with your laptop, is Postfix. From some web searching, I found this page by Michael Prokop. In short, the magic contents of /etc/postfix/main.cf are as follows:
relayhost = mail.vc-graz.ac.at
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/smtp_auth
smtp_sasl_security_options = noanonymous
Note that if your mail relay runs on port 26, you can put the port number on the relayhost line, like this:
relayhost = mail.vc-graz.ac.at:26
You then have to set up /etc/postfix/smtp_auth with your client password information.
Overall, it takes a few minutes to set it all up, but you end up with your laptop being able to send email from pretty much any Internet connection. I'm posting it here in case anyone else finds it convenient.