nodemailer smtp-server giving 421 mydomain.com You talk too soon error

Question

I am running an SMTP Server using http://nodemailer.com/extras/smtp-server/ to accept all the mail submissions.
When the mail submission agent uses STARTTLS I get the following error.

5|producer  | [2020-10-09 07:28:52] DEBUG [#ff7cqlwi7rat6z2k] C: EHLO qa.mydomain.com
5|producer  | [2020-10-09 07:28:52] DEBUG [#ff7cqlwi7rat6z2k] S: 421 mydomain.com You talk too soon
5|producer  | [2020-10-09 07:28:52] INFO  [#ff7cqlwi7rat6z2k] Connection closed to 91.198.201.301

However, this happens only with some clients and I have tried with few other tools and it upgrades the connection to TLS without any issue.

Below are my server configuration options.

SMTPServerOptions = { 
  secure: false,
  hideSTARTTLS:true,
  authOptional: true,
  debug: true,
  logger: true,
  onAuth,
  onData
}

if(conf.tls) {
  SMTPServerOptions.ca = fs.readFileSync('./certificates/chain.pem', 'ascii')
  SMTPServerOptions.key = fs.readFileSync('./certificates/privkey.pem','ascii')
  SMTPServerOptions.cert = fs.readFileSync('./certificates/cert.pem','ascii')
}
//creating new SMTP object
const server = new SMTPServer(SMTPServerOptions);

server.on('error', err => {
  error(err)
  throw err 
});

server.listen(conf.server_port);

Answer 1

Was able to solve this by commenting some parts of the code in the nodemailer smtp-server module. Just posting here so that it would help others who are seeking the answer for the same.

Some SMTP clients like the one which I used do not wait for the server response after the connection and sends the EHLO or HELO command to the server. From the source code of the module, these clients are treated as early talkers and the connection is blocked to avoid spamming.
Commenting the timeout function and emitting connectionReady() event immediately solved the problem.

     /**
     * Initiates the connection. Checks connection limits and reverse resolves client hostname. The client
     * is not allowed to send anything before init has finished otherwise 'You talk too soon' error is returned
     */
    init() {
        // Setup event handlers for the socket
        this._setListeners(() => {
            // Check that connection limit is not exceeded
            if (this._server.options.maxClients && this._server.connections.size > this._server.options.maxClients) {
                return this.send(421, this.name + ' Too many connected clients, try again in a moment');
            }
            // Keep a small delay for detecting early talkers
            //setTimeout(() => this.connectionReady(), 100);

            // no need to detect the early talkers
            this.connectionReady();
        });
    }

Also disable the reverse lookup if it is taking time.

 // disabling the reverse lookup which will solve 'you talk so soon problem'
 this.options.disableReverseLookup = true;