I have recently upgraded to 8.0.1 and I am having similar issues with DKIM.
In my case emails fail when they are sent from a billing system running on a webserver, it is a PHP script that connects to Zimbra using SMTP with SSL.
If I send emails from Zimbra using webmail or desktop they pass.
It does not make any difference if the email is being sent to a local account or to a remote email server for delivery.
From the log file I see an extra line about an anonymous tls connection when the webserver is sending the email, versus desktop/webmail, could this be a problem? Does it need to auth at connect somehow, rather than afterwards?
NOTE: The attatched email sources are complete, however domain names/ips etc have been blanked.