Header is not RFC 5322 compliant

Help
1

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)