After a lot of runaround, here's how I got piping working with Plesk.
- CHMOD the /cli/index.php file to 755 as per the manual.
- Via SSH, log into your server as root. (The freeware WinSCP is excellent for this.)
- Change directory to /var/qmail/mailnames/yourdomain.com/
- Create a file called .qmail-support. (Change 'support' to 'help' or whatever you want your support email to be.)
- Edit the file, adding the following line, WITHOUT THE QUOTES: "| php -q /var/www/vhosts/yourdomain.com/httpdocs/cli/index.php"
- Save the file and CHMOD it to 0644.
That's it! Make sure you've created a mail queue for that address in the Admin control panel, and wooo-hooo! away you go!