add_filter( 'wcfmmp_store_new_order_email_header', function( $headers, $vendor_id ) {
    $vendor_email = wcfm_get_vendor_store_email_by_vendor( $vendor_id );
    if( is_array( $headers ) ) {
        $headers[] = 'From: <' . $vendor_email . '>';
    } else {
        $headers .= 'From: <' . $vendor_email . '>';
    }
    return $headers;
}, 99, 2 );