My previous attempt at sending mail to /dev/null was not successful. The following actually works for me.
I created a new spameater account and have the extension address sent there. In /etc/qmail/users/assign:
Once this was in place I ran ‘qmail-newu’. So far, so good.
The next step was to create a black hole .qmail file in /home/spameater:
# Throw messages away undelivered
To test this I did the following as a regular user:
mail -s "Qmail Black Hole" email@example.com < /dev/null
I reviewed /var/log/maillog and saw that the message was successfully 'delivered'. I also confirmed that spameater's Maildir tree contained no messages. So far, this solution looks acceptable.