When sending an email without SPF signature to a domain hosted on Plesk server, the following message is returned by SPF checker:
CONFIG_TEXT: <firstname.lastname@example.org>: host mail.example.org[203.0.113.3] said: 550
5.7.23 SPF validation failed. Please see
URL contained in the email, pointing to
www.openspf.org, does not work.
openspf.org is hardcoded in the
libspf2 library; however,
openspf.org website is not working anymore. A Plesk item PPPM-11261 has been created to address this issue in future Plesk updates.
As a workaround, set custom SPF explanation text that will be returned by mail server in case of rejection:
Go to Tools & Settings > Mail Server Settings.
Under SPF Spam protection section, specify custom SPF explanation text and click OK.