You must rename "nullmailer-inject" to "nullmailer-inject2" and use this as "nullmailer-inject":

#!/opt/local/bin/python

import sys
import subprocess

args = ['/opt/local/libexec/nullmailer/nullmailer-inject2'] + sys.argv[1:]
proc = subprocess.Popen(args, bufsize=-1,
                        stdin=subprocess.PIPE,
                        stdout=subprocess.PIPE,
                        stderr=subprocess.PIPE,
                        close_fds=True)

first = sys.stdin.readline()
if not (first.startswith('From ') or first.startswith('>From ')):
    proc.stdin.write(first)

for line in sys.stdin:
    proc.stdin.write(line)

stdoutdata, stderrdata = proc.communicate()
if stdoutdata:
    sys.stdout.write(stdoutdata)
if stderrdata:
    sys.stderr.write(stderrdata)

sys.exit(proc.returncode)