Fix #1129

server/notification-providers/smtp.js

@@ -14,16 +14,20 @@ class SMTP extends NotificationProvider {
             secure: notification.smtpSecure,
             tls: {
                 rejectUnauthorized: notification.smtpIgnoreTLSError || false,
-            },
-            dkim: {
+            }
+        };
+
+        // Fix #1129
+        if (notification.smtpDkimDomain) {
+            config.dkim = {
                 domainName: notification.smtpDkimDomain,
                 keySelector: notification.smtpDkimKeySelector,
                 privateKey: notification.smtpDkimPrivateKey,
                 hashAlgo: notification.smtpDkimHashAlgo,
                 headerFieldNames: notification.smtpDkimheaderFieldNames,
                 skipFields: notification.smtpDkimskipFields,
-            }
-        };
+            };
+        }
 
         // Should fix the issue in https://github.com/louislam/uptime-kuma/issues/26#issuecomment-896373904
         if (notification.smtpUsername || notification.smtpPassword) {