Mail to specific address fails on Plesk: marked message to be discarded if not explicitly delivered (discard action)

Symptoms

• Email messages are not delivered to a specific email address on Plesk

• One of the following messages show up in /var/log/maillog:

  hostname dovecot: service=lda, user=jdoe@example.com, ip=[]. sieve: msgid=20180923061540.Horde.zyLIfVJmZISHx3cXgJ9740t@webmail.example.com: marked message to be discarded if not explicitly delivered (discard action)

  hostname dovecot: service=lda, user=jdoe@example.com, ip=[]. sieve: msgid=069001d82be4$a3dc6ee0$eb954ca0$@webmail.example.com: reject action: rejected message from user1@example.net (reject)

• There is a filter in webmail, or .sieve file with discard directives inside the mailbox directory

  # cat /var/qmail/mailnames/example.com/jdoe/.dovecot.sieve
  # rule:[Blockaddress]
  if header :contains "from" "user1@example.net"
  {
  discard;
  }

Cause

A custom filter (sieve rule), created from webmail, is blocking the emails coming from specific sender.

Resolution

Remove the discard rule in the webmail or command-line.