These notes refer primarily to Debian 9 (Stretch).

• Debian Reference - Chapter 4. Authentication

Delete or comment out pam_mail.so in /etc/pam.d/sshd.

# DISABLED: 2018-12-18 - no need on a single user system
# Print the status of the user's mailbox upon successful login.
#session    optional     pam_mail.so standard noenv # [1]