smtp-server

Node.JS module for creating SMTP and LMTP server instances on the fly.

See smtp-server homepage for documentation and terms.

Using the SIZE extension

Set the size option to advertise a maximum message size to clients. Declared sizes in MAIL FROM:<addr> SIZE=nnn parameters are checked against the limit, but the actual transfer size is not enforced by the server itself — message data is streamed to the application, so it is up to the application to decide what to do with an oversized message. Check stream.sizeExceeded in your onData handler to detect oversized messages:

const server = new SMTPServer({
    size: 1024 * 1024, // 1 MiB limit
    onData(stream, session, callback) {
        stream.on('end', () => {
            if (stream.sizeExceeded) {
                const err = new Error('Message too large');
                err.responseCode = 552;
                return callback(err);
            }
            callback(null, 'OK');
        });
        stream.resume(); // Consume the stream
    }
});

The sizeExceeded flag and the byteLength byte counter on the data stream are updated in real time while the message is being received, so a handler that wants to stop an oversized transfer early can also check the flag from a data event instead of waiting for end.