added SMTP monitor (#5489)

Co-authored-by: Frank Elsinga <frank@elsinga.de>

server/monitor-types/smtp.js

@@ -0,0 +1,35 @@
+const { MonitorType } = require("./monitor-type");
+const { UP } = require("../../src/util");
+const nodemailer = require("nodemailer");
+
+class SMTPMonitorType extends MonitorType {
+    name = "smtp";
+
+    /**
+     * @inheritdoc
+     */
+    async check(monitor, heartbeat, _server) {
+        let options = {
+            port: monitor.port || 25,
+            host: monitor.hostname,
+            secure: monitor.smtpSecurity === "secure", // use SMTPS (not STARTTLS)
+            ignoreTLS: monitor.smtpSecurity === "nostarttls", // don't use STARTTLS even if it's available
+            requireTLS: monitor.smtpSecurity === "starttls", // use STARTTLS or fail
+        };
+        let transporter = nodemailer.createTransport(options);
+        try {
+            await transporter.verify();
+
+            heartbeat.status = UP;
+            heartbeat.msg = "SMTP connection verifies successfully";
+        } catch (e) {
+            throw new Error(`SMTP connection doesn't verify: ${e}`);
+        } finally {
+            transporter.close();
+        }
+    }
+}
+
+module.exports = {
+    SMTPMonitorType,
+};