This hook allows control over embedding of images from remote hosts when generating emails. In some circumstances you may not want to embed the remote image – perhaps due to slow performance from the remote server.

function disallowed_host( $allowed, $host ) {
    // return false; // if you want to link rather than embed all remote images
    if ( 's.w.org' === $host ) {
        $allowed = false;
    }
    return $allowed;
}
add_filter( 's2_remote_embed_host', 'disallowed_host', 10, 2 );