Header is not RFC 5322 compliant

I’m getting this error from the dashboard logs when trying to send a test email:

  "email_type": "try_provider_configuration_email",
  "to": "****@****.nl",
  "error": "Mail command failed: 550 From header is missing, header is not RFC 5322 compliant"

I’m using Sendinblue as our STMP server. Not sure what this error means, can someone help?

(i removed my email above)