Notifications from Plesk are bounced: duplicate header field "Content-Type"

Symptoms

• Plesk Notifications are bounced with one of the following error messages:

  CONFIG_TEXT: <mail-reports@example.com>: host smtp.example.com[203.0.113.2] said: 550 Messages
  should have one or no Content-Type headers, not 2. (in reply to end of DATA
  command)

  ---

  CONFIG_TEXT: INVALID HEADER
  Duplicate header field: "Content-Type"

• If Plesk Email Security Extension is installed, the following error might be shown in the log file /var/log/maillog:

  CONFIG_TEXT: amavis[15703]: (15703-13-2) Blocked BAD-HEADER-0 {BouncedInbound,Quarantined}

• Analyzing the mail header of a Plesk Notification, it includes the Content-Type twice:

  CONFIG_TEXT: ...
  Content-Type: text/html;
  charset="UTF-8"
  MIME-Version: 1.0
  Content-Type: text/html;
  charset="UTF-8"
  ...

Cause

Resolution

Workaround

As a workaround, disable the Plesk HTML notifications in Plesk Panel.ini file:

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