BUG with Mailgun integration

Unable to send test email with Mailgun SMTP integration:
5.7.1 [xxx] Our system has detected that this message is 5.7.1 not RFC 5322 compliant: 5.7.1 ‘From’ header is missing. 5.7.1 To reduce the amount of spam sent to Gmail, this message has been 5.7.1 blocked. Please visit 5.7.1 https://support.google.com/mail/?p=RfcMessageNonCompliant 5.7.1 and review RFC 5322 specifications for more information. dk7si3180995ejb.285 - gsmtp

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