diff --git a/src/routes/mailserver.js b/src/routes/mailserver.js
index 41eccd598..8e96dbd81 100644
--- a/src/routes/mailserver.js
+++ b/src/routes/mailserver.js
@@ -46,7 +46,7 @@ async function proxyToMailContainer(port, pathname, req, res, next) {
 
     req.clearTimeout(); // TODO: add timeout to mail server proxy logic instead of this
     mailserverProxy(req, res, function (error) {
-        if (!error) return next();
+        if (!error) return; // response was already sent by proxy, do not proceed to connect-lastmile
 
         if (error.code === 'ECONNREFUSED') return next(new HttpError(424, 'Unable to connect to mail server'));
         if (error.code === 'ECONNRESET') return next(new HttpError(424, 'Unable to query mail server'));