The earlier solution only corrects the sender address in a registration email.
If you hit reply to a comment notification the reply address is actually the poster’s address.
To show the poster’s address in the ‘From’ field of the notification email:
1. Open pluggable-functions.php
2. Change <$wp_admin> in line 290 to: <$comment->comment_author_email>
Voila!
<Geeky Excitement! I’m starting to get PHP! (I’m 90 Pages in to PHP5 and MySQL Bible.)>