I have FreeBSD box running qmail-1.03, vpopmail-5.4.3, qmailadmin-1.0.20, sqwebmail-4.0.6, maildrop-1.6.3

I want users from virtual domains to be able to use maildrop filtering (configured by each of them from sqwebmail and enabled/disabled by me from qmailadmin.

short version of problem:
maildrop fails delivering a message for a virtual domain user when using mail filters (with to or cc rules) because it wants to run qmail-inject using $SHELL: execle(shell, q, "-c", cmd, (const char *)0, env);
$SHELL seen by maildrop in this case is the shell of the vpopmail user

long version:
the file: /usr/local/share/sqwebmail/maildirfilterconfig contains:
MAILDIRFILTER=../.mailfilter
MAILDIR=./Maildir
this allows one to edit mail filtering rules, located in (e.g.) /usr/local/vpopmail/domains/domain.com/postmaster/.mailfilter

but, vdelivermail from /usr/local/vpopmail/domains/domain.com/.qmail-default ignores the .mailfilter
this is why I activated SPAM_DETECTION in qmailadmin, with the SPAM_COMMAND="| preline /usr/local/bin/maildrop .mailfilter"

so far so good. I add a virtual account test@domain.com from qmailadmin, with spam detection enabled. I login from sqwebmail go to "Edit Mail Filters" and I click on save all changes. this creates the file .mailfilter

when I send a test message to the account I get the following error in qmail logs:
delivery xxx: failure: Unable_to_execute_/nonexistent/

Fix: 

quick fix: pw user mod vpopmail -s /bin/sh

fix #1: empower vpopmail user /bin/sh instead of /nonexistent
root@jerry:/usr/ports/mail/vpopmail# diff pkg-install.orig pkg-install
64c64
<               $result = system ("/usr/sbin/pw useradd $user -g $users{$user} -d \"$ENV{PKG_PREFIX}\" -s /nonexistent $x");
---
>               $result = system ("/usr/sbin/pw useradd $user -g $users{$user} -d \"$ENV{PKG_PREFIX}\" -s /bin/sh $x");

fix #2: modify vdelivermail from vpopmail in order to set env variable SHELL to /bin/sh - in case the program (maildrop) needs to run another program (qmail-inject, in case of to or cc rules), using $SHELL env variable

I know fix #2 is not FreeBSD related; I will send this informations to vpopmail mailing-list also.
but, I guess this is what ports is all about: tweaking software in order to run on FreeBSD without problems.