Unable to receive mail when forwarding is configured in webmail interface

Symptoms

• A bounce email like the following is received on the sender mailbox:

  CONFIG_TEXT: This is the mail system at host srv1.afdbayern.de.
  I'm sorry to have to inform you that your message could not
  be delivered to one or more recipients. It's attached below.
  For further assistance, please send mail to postmaster.
  If you do so, please include this problem report. You can
  delete your own text from the attached returned message.
  The mail system
  test@cos-media.de: host example.com[203.0.113.2] said: 550 5.7.1
  Command rejected (in reply to end of DATA command)

• Forwarding is configured in webmail directly.

• Records like the following can be found in /var/log/maillog:

  CONFIG_TEXT: postfix/smtp[9220]: BF5C06167C: to=test@example.com, relay=example.com[203.0.113.2]:25, delay=30, delays=0.01/0.02/30/0.2, dsn=5.7.1, status=bounced (host example.com[203.0.113.2] said: 550 5.7.1 Command rejected (in reply to end of DATA command))

• SPF records are configured properly for the sender email.

• The issue does not occur in sending from another mailbox on the same domain.

Cause

Incorrect forwarding configuration.

Resolution

Configure forwarding via Plesk interface.