add_filter( 'propertyhive_property_enquiry_to', 'customise_enquiry_to', 10, 2 );
function customise_enquiry_to( $to, $property_id )
{
    $user_id = get_post_meta( $property_id, '_frontend_submission_user_id', TRUE );
    $email_address = get_post_meta( $user_id, '_email_address', TRUE );
    if ( $email_address == '' )
    {
        // maybe provide fallback address here
        // return 'fallback@somewhere.com';
    }
    return $email_address;
}