Email messages are not signed with DKIM if they are sent not via SMTP: DKIM Feed: Syntax error

Symptoms 

Emails are not signed with DKIM in case they are sent using mail() PHP function or mail utility, below information can be found in /var/log/maillog:

CONFIG_TEXT: dk_sign[28686]: Starting the dk_sign filter...
dk_sign[28686]: DKIM error: DKIM Feed: Syntax error
plesk sendmail[28683]: Error during 'dd51-domainkeys' handler
postfix/pickup[27254]: 024B213A0AE6: uid=10000 from=<mail@example.com>

OR

CONFIG_TEXT: check-quota[1337]: cannot get sender domain
check-quota[1337]: Unable to intialize check-quota mail handler
plesk sendmail[1381]: Error during 'check-quota' handler
postfix/pickup[6145]: 04655870621: uid=10000 from=user@example.com

As a result mail can be delivered to spam folder.

Cause 

This is Plesk bug with ID PPPM-6690 which is planned to be fixed in future product updates.

Resolution

As a workaround, specify email headers manually or submit a message directly through SMTP server. One of the following PHP scripts can be used as an example:

• Send Email From a PHP Script Using SMTP Authentication
• How do I send PHP mail via SMTP
• SMTP.php