I've been trying for ages to get a simple pipe to work but no luck so far. It's been several days of trying. What I had on my old server (now deleted) was a simple custom rule on an email account. It said Code: to "|/usr/bin/php /path/to/my/script.php" It worked perfectly before but I can't figure it out on the new server. That is Maildrop syntax, which Sieve does not support at all. The default in ISPConfig however is Sieve. I tried to change to Maildrop by changing "virtual_transport" to "maildrop" in /etc/postfix/main.cf. Which results in something like "maildrop: Temporary authentication failure." in /var/log/mail.log. I have to switch the config back to "dovecot" after testing to make my incoming email work again. I even tried the Sieve pipe plugin but unfortunately it's a dead end. Will not compile on Ubuntu 16.04.