Question

My android application uses the JavaMail library to send an email, with an attachment. Over Wifi the application work very well, but when I disconnect from WiFi and turn on 3G, the mail can’t be sent. It seems that I cant reach out to the mail server, even that the mailserver is public on the internet, not locally on the network.

Does the SMTP port changes when I connect to 3G? If so, this is very strange. The codesnippet for sending mail is beneath

package Logic;

public class SendMail extends javax.mail.Authenticator {

    private String _user;
    private String _pass;

    private String[] _to;
    private String _from;

    private String _port;
    private String _sport;

    private String _host;

    private String _subject;
    private String _body;

    private boolean _auth;

    private boolean _debuggable;

    private Multipart _multipart;
    SharedPreferences sharedPrefs;

    public SendMail() {

        _user = ""; // username
        _pass = ""; // password
        _from = ""; // email sent from
        _subject = ""; // email subject
        _body = ""; // email body

        _debuggable = false; // debug mode on or off - default off
        _auth = true; // smtp authentication - default on

        _multipart = new MimeMultipart();

        // There is something wrong with MailCap, javamail can not find a
        // handler for the multipart/mixed part, so this bit needs to be added.
        MailcapCommandMap mc = (MailcapCommandMap) CommandMap
                .getDefaultCommandMap();
        mc.addMailcap("text/html;; x-java-content-handler=com.sun.mail.handlers.text_html");
        mc.addMailcap("text/xml;; x-java-content-handler=com.sun.mail.handlers.text_xml");
        mc.addMailcap("text/plain;; x-java-content-handler=com.sun.mail.handlers.text_plain");
        mc.addMailcap("multipart/*;; x-java-content-handler=com.sun.mail.handlers.multipart_mixed");
        mc.addMailcap("message/rfc822;; x-java-content-handler=com.sun.mail.handlers.message_rfc822");
        CommandMap.setDefaultCommandMap(mc);



    }

    public SendMail(Context c) {
        this();

        sharedPrefs = PreferenceManager.getDefaultSharedPreferences(c);
        _host = sharedPrefs.getString("host", null);
        _port = sharedPrefs.getString("port", null);
        _sport = sharedPrefs.getString("port", null); 
        _user = sharedPrefs.getString("mail", null);
        _pass = sharedPrefs.getString("pw", null); 

    }

    public boolean send() throws Exception {
        Properties props = _setProperties();

        if (!_user.equals("") && !_pass.equals("") && _to.length > 0
                && !_from.equals("") && !_subject.equals("")
                && !_body.equals("")) {

            Session session = Session.getInstance(props,
                    new GMailAuthenticator(_user, _pass));

            MimeMessage msg = new MimeMessage(session);


            msg.setFrom(new InternetAddress(_from));

            InternetAddress[] addressTo = new InternetAddress[_to.length];
            for (int i = 0; i < _to.length; i++) {
                addressTo[i] = new InternetAddress(_to[i]);
            }
            msg.setRecipients(MimeMessage.RecipientType.TO, addressTo);

            msg.setSubject(_subject);
            msg.setSentDate(new Date());

            // setup message body
            BodyPart messageBodyPart = new MimeBodyPart();
            messageBodyPart.setText(_body);
            _multipart.addBodyPart(messageBodyPart);


            // Put parts in message
            msg.setContent(_multipart);

            // send email, henger noe her?!
            Transport.send(msg);

            return true;
        } else {
            return false;
        }
    }

    public void addAttachment(String filename) throws Exception {
        BodyPart messageBodyPart = new MimeBodyPart();
        DataSource source = new FileDataSource(filename);
        messageBodyPart.setDataHandler(new DataHandler(source));
        messageBodyPart.setFileName(filename);


        _multipart.addBodyPart(messageBodyPart);
    }

    class GMailAuthenticator extends Authenticator {
        String user;
        String pw;

        public GMailAuthenticator(String username, String password) {
            super();
            this.user = username;
            this.pw = password;
        }

        public PasswordAuthentication getPasswordAuthentication() {
            return new PasswordAuthentication(user, pw);
        }
    }

    private Properties _setProperties() {
        Properties props = new Properties();

        props.put("mail.smtp.host", _host);
        props.put("mail.smtp.starttls.enable", "true");

        if (_debuggable) {
            props.put("mail.debug", "true");
        }

        if (_auth) {
            props.put("mail.smtp.auth", "true");
        }

        props.put("mail.smtp.port", _port);
        props.put("mail.smtp.socketFactory.port", _sport);
        props.put("mail.smtp.socketFactory.fallback", "true");

        return props;
    }

    public String getBody() {
        return _body;
    }

    public void setBody(String _body) {
        this._body = _body;
    }

    public void setTo(String[] toArr) {
        this._to = toArr;
    }

    public void setFrom(String string) {
        this._from = string;
    }

    public void setSubject(String string) {
        this._subject = string;
    }
}

Yes, I have specified the correct permission in my manifest

<uses-permission android:name="android.permission.INTERNET"/>

Answer 1

Some ISP restrict usage of SMTP servers and force you to use their SMTP servers instead of your own.
So you’d receive mail from pop.yourmailserver.com and send mail using smtp.yourisp.com.

Or, they forbid you to send mail using SMTP from 3G.