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

• This error is logged to /var/log/maillog

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

• There's a filter in webmail, or .sieve file with discard directives inside the mailbox directory (e.g. /var/qmail/mailnames/example.com/john.doe/sieve/)

  Sieve Configuration Example

  # Blacklisted Addresses
  ...
  if address :all :comparator "i;ascii-casemap" :is ["From", "Sender", "Resent-From"] ["john.doe1@example.org", "john.doe@example.com"] {
  discard;
  stop;
  }

  # rule:[.]
  if true
  {
  discard;
  }

Cause

A custom filter is configured to discard messages to this specific recipient.

Resolution

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