prove what?
if the machine with an rDNS of bobhoffman.com sends mail from *@bobhoffman.com, and is the MX of this domain, would anybody think this is a forgery?
Mouss... I mean Ratatouille :-)
Answer: Possibly
Depends on many factors doesn't it?
I know you are on other lists like SA so I am not sure why you are leading us down the infinite possibilities path...
...seeing as you are quite excellent at *nix and *net administration and implementations.
:-)
- rh