Outgoing mail messages are not being DKIM signed in Plesk 18.0.80 with Qmail

Symptoms

• Outgoing mail messages are either not received or are marked as spam by remote mail services.
• The mail log contains the following record regarding DKIM signing:

PLESK_INFO: Jan 1 00:00:00 host dk_sign[123456]: 123456: Not sendmail, not authorized, TCPREMOTEIP=NULL not whitelisted - not signing
Jan 1 00:00:00 host qmail-remote[123456]: 123456: dk_sign: stderr: SKIP

• Plesk version is 18.0.80 or lower.

Cause

This was confirmed to be a bug identified as PPPM-15503.

Resolution

Update Plesk to 18.0.81 or temporarily use Postfix.