and Message-ID tightened up recently rules of incoming messages related to Message-ID parameter in message headers.

Following RFC 5322, every message has to contain the Message-ID parameter in its headers. This parameter's value should have correct syntax "string@domain", see mailservers check whether messages contain the Message-ID parameter and whether it has correct syntax. Otherwise messages can be rejected with the error "550-5.7.1 Messages missing a valid messageId header are not accepted."

The Messages-ID parameter is inserted into the message headers by Email client, application or script that generate the message. If you get the error above, check settings of your application, script or email client, or you can use another email client. This check is matter of mailservers and we have no possibility to influence it. The server doesn't make any changes of the Message-ID parameter because of DKIM sign, that could be invalid after any changes of message headers.