javax.mail.MessagingException: Connect failed;
nested exception is:
javax.net.ssl.SSLHandshakeException:
sun.security.validator.ValidatorException: PKIX path building failed:
sun.security.provider.certpath.SunCertPathBuilderException: unable to find
valid certification path to requested target



package emailattachmentsdownloader;
import java.io.File;
import java.io.IOException;
import java.util.Properties;
import javax.mail.Address;
import javax.mail.Folder;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.NoSuchProviderException;
import javax.mail.Part;
import javax.mail.Session;
import javax.mail.Store;
import javax.mail.internet.MimeBodyPart;
/**
 * This program demonstrates how to download e-mail messages and save
 * attachments into files on disk.
 *
 *
 *
 */
public class EmailAttachmentsDownloader {
private String saveDirectory;
/**
 * Sets the directory where attached files will be stored.
 * @param dir absolute path of the directory
 */
public void setSaveDirectory(String dir) {
    this.saveDirectory = dir;
}
/**
 * Downloads new messages and saves attachments to disk if any.
 * @param host
 * @param port
 * @param userName
 * @param password
 */
public void downloadEmailAttachments(String host, String port,
        String userName, String password) {
    Properties properties = new Properties();

    // server setting
    properties.put("mail.pop3.host", host);
    properties.put("mail.pop3.port", port);

    // SSL setting
    properties.setProperty("mail.pop3.socketFactory.class",
            "javax.net.ssl.SSLSocketFactory");
    properties.setProperty("mail.pop3.socketFactory.fallback", "false");
    properties.setProperty("mail.pop3.socketFactory.port",
            String.valueOf(port));

    Session session = Session.getDefaultInstance(properties);

    try {
        // connects to the message store
        Store store = session.getStore("pop3");
        store.connect(userName, password);

        // opens the inbox folder
        Folder folderInbox = store.getFolder("INBOX");
        folderInbox.open(Folder.READ_ONLY);

        // fetches new messages from server
        Message[] arrayMessages = folderInbox.getMessages();

        for (int i = 0; i < arrayMessages.length; i++) {
            Message message = arrayMessages[i];
            Address[] fromAddress = message.getFrom();
            String from = fromAddress[0].toString();
            String subject = message.getSubject();
            String sentDate = message.getSentDate().toString();

            String contentType = message.getContentType();
            String messageContent = "";

            // store attachment file name, separated by comma
            String attachFiles = "";

            if (contentType.contains("multipart")) {
                // content may contain attachments
                Multipart multiPart = (Multipart) message.getContent();
                int numberOfParts = multiPart.getCount();
                for (int partCount = 0; partCount < numberOfParts; partCount++) {
                    MimeBodyPart part = (MimeBodyPart) multiPart.getBodyPart(partCount);
                    if (Part.ATTACHMENT.equalsIgnoreCase(part.getDisposition())) {
                        // this part is attachment
                        String fileName = part.getFileName();
                        attachFiles += fileName + ", ";
                        part.saveFile(saveDirectory + File.separator + fileName);
                    } else {
                        // this part may be the message content
                        messageContent = part.getContent().toString();
                    }
                }

                if (attachFiles.length() > 1) {
                    attachFiles = attachFiles.substring(0, attachFiles.length() - 2);
                }
            } else if (contentType.contains("text/plain")
                    || contentType.contains("text/html")) {
                Object content = message.getContent();
                if (content != null) {
                    messageContent = content.toString();
                }
            }

            // print out details of each message
            System.out.println("Message #" + (i + 1) + ":");
            System.out.println("t From: " + from);
            System.out.println("t Subject: " + subject);
            System.out.println("t Sent Date: " + sentDate);
            System.out.println("t Message: " + messageContent);
            System.out.println("t Attachments: " + attachFiles);
        }

        // disconnect
        folderInbox.close(false);
        store.close();
    } catch (NoSuchProviderException ex) {
        System.out.println("No provider for pop3.");
        ex.printStackTrace();
    } catch (MessagingException ex) {
        System.out.println("Could not connect to the message store");
        ex.printStackTrace();
    } catch (IOException ex) {
        ex.printStackTrace();
    }
}

/**
 * Runs this program with Gmail POP3 server
 */
public static void main(String[] args) {
    String host = "pop.gmail.com";
    String port = "995";
    String userName = "****";
    String password = "****";

    String saveDirectory = "D:/Attachments";

    EmailAttachmentsDownloader receiver = new EmailAttachmentsDownloader();
    receiver.setSaveDirectory(saveDirectory);
    receiver.downloadEmailAttachments(host, port, userName, password);

}