50 min after the test email message I mentioned above (one line "help" in plain text message body, Subject ignored), I received an email reply from the script, so piping worked. I don't know why the delay (the delay is not because of receiving mailserver, I whitelisted johnny IP-address there). Forwarding isn't visible in Plesk. A copy of the incoming email message is kept in the mailbox, but that's no problem.
johnny.heliohost.org resolves to 65.19.141.67, but PTR for that IP-address doesn't resolve:
dig -x 65.19.141.67 @8.8.8.8
...
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 23760
...
dig +trace -x 65.19.141.67
...
67.141.19.65.in-addr.arpa. 86400 IN CNAME 67.subnet64.141.19.65.in-addr.arpa.
subnet64.141.19.65.in-addr.arpa. 86400 IN NS ns2.heliohost.org.
subnet64.141.19.65.in-addr.arpa. 86400 IN NS ns1.heliohost.org.
;; Received 129 bytes from 216.66.1.2#53(ns4.he.net) in 36 ms
dig 67.141.19.65.in-addr.arpa. ptr @ns1.heliohost.org
...
;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 13410
...
dig 67.141.19.65.in-addr.arpa. ptr @ns2.heliohost.org
...
;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 51684
...
This is bad for emails from johnny.
Thank you.