Send emails with SMTP protocol using socket class

I'm trying to develop a class that sends email through the SMTP protocol but without using libraries that facilitate sending mail such as MimeMessage or MailUtility. This is what I've done so far:

public class MailSender {

    protected String localMachine;
    protected String localSMTPServer;
    private String textMail;

    private String mailFrom;

    public MailSender(String localSMTPServer, String mailFrom) {
        try {
            this.textMail = "null";

            this.localSMTPServer = localSMTPServer;

            this.mailFrom = mailFrom;

            InetAddress localIP = InetAddress.getLocalHost();

            this.localMachine = localIP.getHostAddress();

        } catch (UnknownHostException ex) {
            Logger.getLogger(MailSender.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
     //getter and setter
    public void smtp(String command) {
        try {
            System.out.println(sIn.readLine());
             os.write((command   "\n").getBytes());
             System.out.println("end");
        } catch (IOException ex) {
            System.out.println(ex);
            Logger.getLogger(MailSender.class.getName()).log(Level.SEVERE, null, ex);
        }
       
    }
 BufferedReader sIn;
 OutputStream os;
    public Boolean trySend(String destUser, String subject, String message) {
        Socket socket;
        try {
            socket = new Socket(localSMTPServer, 587);

            OutputStreamWriter sOutW = new OutputStreamWriter(socket.getOutputStream(), "ASCII");

            PrintWriter sOut = new PrintWriter(sOutW, true);
            InputStreamReader sInR = new InputStreamReader(socket.getInputStream(), "ASCII");
            
            sIn = new BufferedReader(sInR);
            os = socket.getOutputStream();
            this.nextSMTPCode(sIn, 220);
            sOut.println("EHLO");
            this.nextSMTPCode(sIn, 250);
      
            sOut.println("AUTH LOGIN");
            this.nextSMTPCode(sIn, 250);
            
            sOut.println("xxxxxxxxxxxxxxx=="); //username 
            sOut.println("xxxxxxxxxxxxxx="); //password 
            this.nextSMTPCode(sIn, 250);
            
            sOut.println("MAIL FROM: <"   this.mailFrom   ">");
            this.nextSMTPCode(sIn, 250);
            sOut.println("RCPT TO: <"   destUser   ">");
            this.nextSMTPCode(sIn, 250);
            
            
            
            sOut.println("DATA");
            this.nextSMTPCode(sIn, 250);
            sOut.println("From: "   this.mailFrom);
            sOut.println("To: "   destUser);
            sOut.println("Subject: "   subject);
            sOut.println(message);
            
            sOut.println(".");
            this.nextSMTPCode(sIn, 250);
            
            sOut.println("QUIT");
            this.nextSMTPCode(sIn, 250);
            
            sIn.close();
            textMail = "From "   this.mailFrom   " To "   destUser   "\n"   "Subject: "   subject   "\nMessage: "   message;
            return true;
        } catch (IOException ex) {
            Logger.getLogger(MailSender.class.getName()).log(Level.SEVERE, null, ex);
        }

        return false;
    }

    public String getMail() {

        return textMail;
    }

    private void nextSMTPCode(BufferedReader sIn, int expectedCode) throws IOException {

        int code;
        String str = sIn.readLine();
        System.out.println(str);
        String sCode = str.substring(0, 3);
        code = Integer.parseInt(sCode);

        if (code != expectedCode) {

            sIn.close();
            System.out.println("Code: "   code   " expected code was: "   expectedCode);
            throw new IOException();
        }

    }

}

I am having problem with communication with smtp server, I am using smtp2go as smtp server. The problem is that I keep getting the 250 code even when this shouldn't show up, finally the email isn't sent. This is a sample communication:

220 mail.smtp2go.com 
ESMTP Exim 4.96-S2G Tue, 10 Jan 2023 14:42:02
 0000 250-mail.smtp2go.com Hello  [xxxxx] 
250-SIZE 52428800 
250-8BITMIME 
250-DSN 250-PIPELINING 
250-PIPECONNECT 
250-AUTH CRAM-MD5
PLAIN LOGIN 
250-CHUNKING

I then get 250-SIZE 52428800 and the other responses (always with the code 250) for every command I send. I tried to telnet and everything seems to work, the problem is in my code but I can't figure out what I'm doing wrong. Does anyone know the cause of this problem?

CodePudding user response：

SMTP can have multiline responses, but you only expect singleline responses. Multiline responses are typical for EHLO. To cite from RFC 5231:

The format for multiline replies requires that every line, except the last, begin with the reply code, followed immediately by a hyphen, "-" (also known as minus), followed by text. The last line will begin with the reply code, followed immediately by , optionally some text, and . As noted above, servers SHOULD send the if subsequent text is not sent, but clients MUST be prepared for it to be omitted.

For example:
 250-First line
 250-Second line
 250-234 Text beginning with numbers
 250 The last line