mail: do port 25 connectivity check with ipv4

src/mail.js

@@ -179,7 +179,7 @@ async function checkOutboundPort25() {
     return await new Promise((resolve) => {
         const client = new net.Socket();
         client.setTimeout(5000);
-        client.connect(25, constants.PORT25_CHECK_SERVER);
+        client.connect({ port: 25, host: constants.PORT25_CHECK_SERVER, family: 4 }); // family is 4 to keep it predictable
         client.on('connect', function () {
             relay.status = true;
             relay.value = 'OK';