Hello,
we have Plesk 17.5.3 Update 55 on a centos 7.5.1804 Server an a problem with incoming mails.
We have checked the dmarc-Check for incoming mails. So from some persons the mail wouldnt received to the mailbox and in the maillog you will see:
Sep 6 15:59:32 xxx dk_check[56435]: Starting...