Classified doesn't send emails (hosting restrictions - need to change form)

Hello, our problem is this: Classifieds doesn't send emails to users from another users via plugin's contact form. It's because of hosting restrictions - email sender has to email address within domain. So user's email, who is sending the email to seller (= responding to another user's classified), has to be set to email header "Reply to".
Can you help me with modifying form code to make it work?
Thanks.