I found myself in a situation where I needed a local mailer that was required to return random exit values0 for email directed to certain users. Luckily, most of the job can be done via your .mc file:
MAILER_DEFINITIONS
Mrandom, P=/opt/bin/random,
F=lsDFMqbE,
S=ruleset_noop, R=ruleset_random,
T=DNS/RFC822/X-Unix,
A=random $u
LOCAL_CONFIG
C{Random} adamo yiorgos admin system operator
Kcomp arith
LOCAL_RULESETS
Sruleset_noop
# This ruleset does nothing
Sruleset_random
# EX__BASE == 64 and EX__MAX == 78. EX_OK == 0, so we mask it to 79.
R$* $: $1 . $(comp r $@ 64 $@ 79 $)
R$* . 79 $@ $1 . 0
LOCAL_RULE_0
R$={Random} < $=w . > $* $#random $: $1
When $#random is called, ruleset_random is called with $1 (the username) as an argument and returns $1.number, where number is a random number between EX__BASE (64) and EX__MAX (78) or zero. Therefore, the $#random binary is executed with $1.number as an argument.
/opt/bin/random can be as simple as this shell script1:
#!/bin/sh
# This is an example (non) delivery agent and is not considered safe to run on a
# production server.
status=`echo $1 | sed -e 's/.*\.\(.*\)$/\1/'`
while read x
do
# noop
done
exit $status
A random EX_* value is returned to the sender of the email, assuming s/he has emailed a local user contained in class $={Random}.
[0] – Valid sendmail exit codes (EX_*) are defined in sysexits.h.
[1] – In my case I wrote the mail delivery agent in C, as I needed to do a few more stuff prior to calling exit().
Managing organized complexity 