Unable to receive external Email on a Plesk hosted mailbox: from unknown[2001:db8:f61:a1ff:0:0:0:80]: 5.7.23 SPF validation failed

Symptoms

• Email cannot be delivered to john.doe@example.net. The following entries are found in /var/log/maillog.

  CONFIG_TEXT: postfix/smtpd[1485479]: connect from unknown[2001:db8:f61:a1ff:0:0:0:80]
  postfix/smtpd[1485479]: ADAE01BA122D: client=unknown[2001:db8:f61:a1ff:0:0:0:80]
  psa-pc-remote[2888394]: ADAE01BA122D: from=jane.doe@example.com to=john.doe@example.net
  ...
  psa-pc-remote[2888394]: ADAE01BA122D: spf: stderr: DATA REPLY:550:5.7.23 SPF validation failed. : Reason: mechanism
  psa-pc-remote[2888394]: ADAE01BA122D: spf: stderr: REJECT
  postfix/cleanup[1485551]: ADAE01BA122D: milter-reject: END-OF-MESSAGE from unknown[2001:db8:f61:a1ff:0:0:0:80]: 5.7.23 SPF validation failed. : Reason: mechanism; from=jane.doe@example.com to=john.doe@example.net proto=ESMTP helo=<mail.example.net>

• SPF spam protection to check incoming mail is enabled under Plesk > Tools & Settings > Mail Settings with fail/deny SPF checking mode.

Cause

Sender SPF record does not contain 2001:db8:f61:a1ff:0:0:0:80 as permitted sender.

Resolution

Sender should adjust the DNS SPF record to allow the IPv6 address from the maillog error or change the IP address from which outgoing emails are sent to match the one from the current SPF record.

Additional Information

What is SPF and how to configure it on a Plesk server
How to change the IP address for outgoing emails in Plesk for Linux with Postfix mail server