/usr/lib/qmail/boot/proc is in qmail 1.06-6.
This file is owned by root:qmail, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #!/bin/sh
# Using splogger to send the log through syslog.
# Using procmail to deliver messages to /var/spool/mail/$USER by default.
exec env - PATH="/var/lib/qmail/bin:$PATH" \
qmail-start '|preline procmail' splogger qmail
|