This tool analyses the headers of an email you paste into it, explaining each Received line. It is a fairly crude tool, and its suggestions are often wrong (though it tries to be conservative in its statements). Don't rely blindly on the results.
It also opens a mail send window, including a quoted copy of the email you pasted in.
When a mail send window is open you can add email addresses to it by right-clicking on an email address or hostname in another window.
This can be configured using the startup script