Avoid spamming the administrator when the daemon is stuck.

lib/worker.js

@@ -504,13 +504,27 @@ function updateWalletStats() {
 
 }
 
+let lastBlockCheckIsOk = true;
 function monitorNodes() {
     global.coinFuncs.getLastBlockHeader((err, block) => {
         if (err !== null) {
-            return global.support.sendEmail(
+            if (lastBlockCheckIsOk) {
+                lastBlockCheckIsOk = false;
+                global.support.sendEmail(
+                    global.config.general.adminEmail,
+                    'Failed to query daemon for last block header',
+                    `The worker failed to return last block header. Please verify if the daemon is running properly.`
+                );
+            }
+            return
+        }
+        if (!lastBlockCheckIsOk) {
+            lastBlockCheckIsOk = true;
+            global.support.sendEmail(
                 global.config.general.adminEmail,
-                'Failed to query daemon for last block header',
-                `The worker failed to return last block header. Please verify if the daemon is running properly.`
+                'Quering daemon for last block header is back to normal',
+                `An warning was sent to you indicating that the the worker failed to return the last block header.
+                 The issue seems to be solved now.`
             );
         }
         const sql = 'SELECT blockID, hostname, ip FROM pools WHERE last_checkin > DATE_SUB(NOW(), INTERVAL 30 MINUTE)';
@@ -525,6 +539,7 @@ function monitorNodes() {
                 }
             })
         });
+        
     });
 }