fix: unref timers to allow graceful process exit

Unref all setTimeout timers so they don't prevent the Node.js process
from exiting when all socket connections are closed. This allows the
server to shut down cleanly in test environments and improves graceful
shutdown behavior in production.

Author: andris9

lib/smtp-connection.js

@@ -207,7 +207,9 @@ class SMTPConnection extends EventEmitter {
             }
 
             // Keep a small delay for detecting early talkers
-            setTimeout(() => this.connectionReady(), 100);
+            let readyTimer = setTimeout(() => this.connectionReady(), 100);
+            // Unref timer so connection init delay doesn't prevent process exit
+            readyTimer.unref();
         });
     }
 
@@ -308,6 +310,8 @@ class SMTPConnection extends EventEmitter {
             greetingSent = true;
             reverseCb(new Error('Timeout'));
         }, 1500);
+        // Unref timer so DNS timeout doesn't prevent process exit
+        reverseTimer.unref();
 
         // Helper function to handle resolver results consistently
         const handleResolverResult = (...args) => {

lib/smtp-server.js

@@ -172,6 +172,9 @@ class SMTPServer extends EventEmitter {
                 return realCallback();
             }
         }, timeout);
+
+        // Unref timer so it doesn't prevent process exit if all connections close naturally
+        this._closeTimeout.unref();
     }
 
     /**