Edit /etc/nullmailer/remotes
and add the following line:
freerelay.err.no smtp --port=587 --starttls x509certfile=/etc/nullmailer/freerelay-error-no-cert.pem x509keyfile=/etc/nullmailer/freerelay-error-no-key.pem
After doing that, restart nullmailer and check that relaying works. Freerelay listens to port 25, 587 (submission), 6667 (irc) and 2525, so if any of those are blocked in your network, try another one.