Remove the headers:
https://practical365.com/exchange-server/remove-internal-exchange-server-names-ip-addresses-message-headers/
Then ensure the send connector FQDN is set to your domain
https://learn.microsoft.com/en-us/powershell/module/exchange/set-sendconnector?view=exchange-ps
set-sendconnector <Internet Send Connector> -Fqdn YourOfficialDomain.com
This assumes the cert assigned to SMTP contains your domain as a subject name