Thread: DEBUG SMTP: could not connect to host "host.domain.uk", port: 25, response: 421

I am using the java mail libary to try and send a confirmation email at the end of my program to say that everything went ok.

I am getting the error 'DEBUG SMTP: could not connect to host "host.domain.uk", port: 25, response: 421'

Here's some code;

    private static void emaillog(){
        // SUBSTITUTE YOUR EMAIL ADDRESSES HERE!!!
        String to = "admin@domain";
        String from = "system@domain";
        // SUBSTITUTE YOUR ISP'S MAIL SERVER HERE!!!
        String host = "smtp.domain.uk";
 
        // Create properties for the Session
        Properties props = new Properties();
 
        // If using static Transport.send(),
        // need to specify the mail server here
        props.put("mail.smtp.host", host);
        // To see what is going on behind the scene
        props.put("mail.debug", "true");
 
        // Get a session
        Session session = Session.getInstance(props);
 
        try {
            // Get a Transport object to send e-mail
            Transport bus = session.getTransport("smtp");
 
            // Connect only once here
            // Transport.send() disconnects after each send
            // Usually, no username and password is required for SMTP
            bus.connect();
            // bus.connect("smtp.domain.uk", "admin", emailPassword);
 
            // Instantiate a message
            Message msg = new MimeMessage(session);
 
            // Set message attributes
            msg.setFrom(new InternetAddress(from));
            InternetAddress[] address = {new InternetAddress(to)};
            msg.setRecipients(Message.RecipientType.TO, address);
            // Parse a comma-separated list of email addresses. Be strict.
            msg.setRecipients(Message.RecipientType.CC,
                                InternetAddress.parse(to, true));
            // Parse comma/space-separated list. Cut some slack.
            msg.setRecipients(Message.RecipientType.BCC,
                                InternetAddress.parse(to, false));
 
            msg.setSubject("Test E-Mail through Java");
            msg.setSentDate(new Date());
 
            // Set message content and send
            setTextContent(msg);
            msg.saveChanges();
            bus.sendMessage(msg, address);
 
            bus.close();
 
        }
        catch (MessagingException mex) {
            // Prints all nested (chained) exceptions as well
            mex.printStackTrace();
            // How to access nested exceptions
            while (mex.getNextException() != null) {
                // Get next exception in chain
                Exception ex = mex.getNextException();
                ex.printStackTrace();
                if (!(ex instanceof MessagingException)) break;
                else mex = (MessagingException)ex;
            }
        }
    }

I have a multifunction printer that emails successfully with these settings

Reception Protocol : SMTP
E-mail Reception Interval : On
: 15minute(s)
Max. Reception E-mail Size : 2 MB
E-mail Storage in Server : Off


SMTP

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SMTP Server Name : smtp.domain.uk
SMTP Port No. : 25
SMTP Authentication : Off
SMTP Auth. E-mail Address :
SMTP Auth. Encryption : Auto Select


POP before SMTP

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POP before SMTP : Off
POP E-mail Address :
Timeout setting after POP Auth. : 300 milli-second


POP3/IMAP4

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POP3/IMAP4 Server Name :
POP3/IMAP4 Encryption : Auto Select


E-mail Communication Port

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POP3 Reception Port No. : 110
IMAP4 Reception Port No. : 143