APT
Disable new notifications from listchanges
To disable the sending of listchanges information from APT run the following command:
dpkg-reconfigure apt-listchanges
Then select None as Method to be used to display changes. Confirm all other requests with the default selected value.
Disable debsecan (Debian Security) notifications
If enabled the debsecan-package will send a daily email with all vulnerabilities in installed packages on the system. To disable remove the MAILTO from the file /etc/cron.d/debsecan.