I'd like a spamc invocation suitable for simple use with Postfix's after-queue content filtering via its 
"pipe" daemon. I've described its expectations at <http://wiki.apache.org/spamassassin/
IntegratedSpamdInPostfix>. The short version is this:

1. If spamc returns 0, Postfix assumes spamc has accepted responsibility for delivering the message.
2. If spamc returns EX_TEMPFAIL or EX_OSERR, Postfix defers the message.
3. Otherwise, Postfix bounces the message.

Given #1, any direct invocation of spamc needs the "-e" argument to pass to sendmail.

I want to defer messages when spamd is unavailable, not deliver or bounce. When using -e, none of 
spamc's various return code options do this:

$ echo foo | spamc -t 1 -U /nonexistent -e /bin/cat; echo $?
foo
0
$ echo foo | spamc -x -t 1 -U /nonexistent -e /bin/cat; echo $?
foo
0
$ echo foo | spamc -E -t 1 -U /nonexistent -e /bin/cat; echo $?
foo
0
$ echo foo | spamc -E -x -t 1 -U /nonexistent -e /bin/cat; echo $?
foo
0

My workaround is to launch spamc through a script, keep the message in a tempfile, and then launch 
sendmail myself. (I pasted such a script into the wiki page.) It works, but it's awkward and less efficient. 
Postfix documentation says:

"With the shell script as shown above you will lose a factor of four in Postfix performance for transit 
mail that arrives and leaves via SMTP. You will lose another factor in transit performance for each 
additional temporary file that is created and deleted in the process of content filtering."