Concatenating the email address to the body before it gets sent.
If you want to give that a try, here’s what you need to change in your “contact_us.php” file:
Find:
Oscommerce Contact form issuesif (tep_validate_email($email_address)) {
tep_mail(STORE_OWNER, STORE_OWNER_EMAIL_ADDRESS, EMAIL_SUBJECT, $enquiry, $name, $email_address);