I know how to do this by analysing the headers, but we have a customers who doesn’t believe what we are telling them.

Is there some kind of 3rd party service or script that we can point them to? So they can send an email to the service (or paste headers, or whatever) and it tells them who is the sending mail server hostname/ip?

(The problem is that this customer doesn’t understand that all email sent from the server uses the same SMTP server hostname/ip. They are convinced that if they connect to "mail.their domain.com" then this is what is reported as the sending mail server/IP.)

Thanks!