replace with custom superagent based on fetch API

src/routes/mailserver.js

@@ -39,7 +39,7 @@ async function proxyToMailContainer(port, pathname, req, res, next) {
     };
 
     const sftpReq = http.request(opts, function (sftpRes) {
-        res.writeHead(sftpRes.statusCode, sftpRes.headers);
+        res.writeHead(sftpRes.status, sftpRes.headers);
         sftpRes.on('error', (error) => next(new HttpError(500, `mailserver error: ${error.message} ${error.code}`)));
         sftpRes.on('end', () => next());
         sftpRes.pipe(res);