hmm, i dont know how this sendmail thing works, but when i was playing around with SR's anonimailer which uses winsock and shit, i couldnt get it to send any emails becuase nowadays most prohibit relaying. but when i finaly found a server that worked it also did the same thing with my IP in the email address. so i tried a few other servers and i finaly found one that displayed the from address like i wanted.
basicaly, i think its how the server handels the from address and it varies depending on the server. if you can figure out how, try to change the server that sendmail uses. here are a few servers that i found support relaying and display the from address correctly: