SMTPError is a rather an interface than real class

types/nodemailer/lib/smtp-connection.d.ts

@@ -70,13 +70,15 @@ declare namespace SMTPConnection {
         dsn?: DSNOptions;
     }
 
-    class SMTPError extends Error {
+    interface SMTPError extends NodeJS.ErrnoException {
         /** string code identifying the error, for example ‘EAUTH’ is returned when authentication */
-        code: string;
+        code?: string;
         /** the last response received from the server (if the error is caused by an error response from the server) */
-        response: string;
+        response?: string;
         /** the numeric response code of the response string (if available) */
-        responseCode: number;
+        responseCode?: number;
+        /** command which provoked an error */
+        command?: string;
     }
 
     interface SentMessageInfo {

types/nodemailer/nodemailer-tests.ts

@@ -1036,9 +1036,10 @@ namespace smtp_connection_test {
             if (err) throw err;
             connection.send({ from: 'a@example.com', to: 'b@example.net' }, 'message', (err, info) => {
                 if (err) {
-                    const code: string = err.code;
-                    const response: string = err.response;
-                    const responseCode: number = err.responseCode;
+                    const code: string = err.code || '???';
+                    const response: string = err.response || '???';
+                    const responseCode: number = err.responseCode || 0;
+                    const command: string = err.command || '???';
                     throw err;
                 }
                 connection.reset(() => {