Seems like it has no relation with Mumara update if powermta is responsible to sign the outgoing email. I would suggest sending an email to Gmail or Yahoo and then check in headers if the DKIM keys are passed there. I am not fully confident about mail-tester results sometimes. Moreover, also check if powermta is signing the outgoing emails, then the switch "Sign Outgoing Emails" in Mumara on the Sending Domain page should be turned off.