A mail server will always add a "Recieved:" line to each mail. Even if you clean it from your system (by recompiling sendmail, or using libmilter), the next server upstreams from you will add a recieved line which says where it got the mail from, ie. your mailserver. That way, the recipient can trace the mail back to the last offending server, and can add that to it's black lists (I know I would. Cleaning Recieved:-headers is a great way to create mailloops, which can take down a mailsystem easily).

Oh, and by the way, you are writing a script that will fake emailaddresses, and you really believe people will give you their real email addresses? When there are 100's of yahoo's and hotmail's and spray's and other services you never heard of?