Embora eu esteja tentando enviar um email na porta 25 sem SSL, estou recebendo um SSLHandshakeException. Abaixo, você pode ver as mensagens de depuração impressas por javax.mail.Session e SSLHandshakeException.

Você pode me ajudar a entender o que está acontecendo?

22 Dec 2009 15:13:21,054  INFO root:197 - DEBUG: setDebug: JavaMail version 1.4ea
22 Dec 2009 15:13:21,059  INFO root:197 - DEBUG: getProvider() returning javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Sun Microsystems, Inc]
22 Dec 2009 15:13:21,103  INFO root:197 - DEBUG SMTP: useEhlo true, useAuth false
22 Dec 2009 15:13:21,103  INFO root:197 - DEBUG SMTP: trying to connect to host "mail.abcdef.com", port 25, isSSL false
22 Dec 2009 15:13:21,338  INFO root:197 - 220 mail.uvwxyz.com ESMTP abcdef Mail Server v8.0; Tue, 22 Dec 2009 15:12:10 -0700
22 Dec 2009 15:13:21,338  INFO root:197 - DEBUG SMTP: connected to host "mail.abcdef.com", port: 25
22 Dec 2009 15:13:21,342  INFO root:197 - EHLO fsb160
22 Dec 2009 15:13:21,383  INFO root:197 - 250-mail.uvwxyz.com Hello fsb160 [65.124.157.54], pleased to meet you.
250-ENHANCEDSTATUSCODES
250-SIZE
250-EXPN
250-ETRN
250-ATRN
250-DSN
250-CHECKPOINT
250-8BITMIME
250-AUTH CRAM-MD5 DIGEST-MD5 LOGIN PLAIN
250-AUTH=LOGIN
250-STARTTLS
250 HELP
22 Dec 2009 15:13:21,383  INFO root:197 - DEBUG SMTP: Found extension "ENHANCEDSTATUSCODES", arg ""
22 Dec 2009 15:13:21,383  INFO root:197 - DEBUG SMTP: Found extension "SIZE", arg ""
22 Dec 2009 15:13:21,384  INFO root:197 - DEBUG SMTP: Found extension "EXPN", arg ""
22 Dec 2009 15:13:21,384  INFO root:197 - DEBUG SMTP: Found extension "ETRN", arg ""
22 Dec 2009 15:13:21,384  INFO root:197 - DEBUG SMTP: Found extension "ATRN", arg ""
22 Dec 2009 15:13:21,384  INFO root:197 - DEBUG SMTP: Found extension "DSN", arg ""
22 Dec 2009 15:13:21,385  INFO root:197 - DEBUG SMTP: Found extension "CHECKPOINT", arg ""
22 Dec 2009 15:13:21,385  INFO root:197 - DEBUG SMTP: Found extension "8BITMIME", arg ""
22 Dec 2009 15:13:21,385  INFO root:197 - DEBUG SMTP: Found extension "AUTH", arg "CRAM-MD5 DIGEST-MD5 LOGIN PLAIN"
22 Dec 2009 15:13:21,385  INFO root:197 - DEBUG SMTP: Found extension "AUTH=LOGIN", arg ""
22 Dec 2009 15:13:21,386  INFO root:197 - DEBUG SMTP: Found extension "STARTTLS", arg ""
22 Dec 2009 15:13:21,386  INFO root:197 - DEBUG SMTP: Found extension "HELP", arg ""
22 Dec 2009 15:13:21,389  INFO root:197 - STARTTLS
22 Dec 2009 15:13:21,430  INFO root:197 - 220 2.0.0 Ready to start TLS
22 Dec 2009 15:13:21,809  INFO root:197 - EHLO fsb160
22 Dec 2009 15:13:21,886  WARN E:314 - Failed to send email
javax.mail.MessagingException: Can't send command to SMTP host;
  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
    at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1420)
    at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1408)
    at com.sun.mail.smtp.SMTPTransport.ehlo(SMTPTransport.java:847)
    at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:384)
    at javax.mail.Service.connect(Service.java:275)
    at javax.mail.Service.connect(Service.java:156)
    at java.lang.Thread.run(Unknown Source)
Caused by: 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
    at com.sun.net.ssl.internal.ssl.Alerts.getSSLException(Unknown Source)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.fatal(Unknown Source)
    at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Unknown Source)
    at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Unknown Source)
    at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(Unknown Source)
    at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(Unknown Source)
    at com.sun.net.ssl.internal.ssl.Handshaker.processLoop(Unknown Source)
    at com.sun.net.ssl.internal.ssl.Handshaker.process_record(Unknown Source)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(Unknown Source)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(Unknown Source)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.writeRecord(Unknown Source)
    at com.sun.net.ssl.internal.ssl.AppOutputStream.write(Unknown Source)
    at com.sun.mail.util.TraceOutputStream.write(TraceOutputStream.java:101)
    at java.io.BufferedOutputStream.flushBuffer(Unknown Source)
    at java.io.BufferedOutputStream.flush(Unknown Source)
    at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1418)
    ... 7 more
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
    at sun.security.validator.PKIXValidator.doBuild(Unknown Source)
    at sun.security.validator.PKIXValidator.engineValidate(Unknown Source)
    at sun.security.validator.Validator.validate(Unknown Source)
    at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.validate(Unknown Source)
    at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkServerTrusted(Unknown Source)
    at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkServerTrusted(Unknown Source)
    ... 19 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
    at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(Unknown Source)
    at java.security.cert.CertPathBuilder.build(Unknown Source)
    ... 25 more

O código usado para criar a sessão e se conectar ao servidor:

Session session = EmailUtilities.createSession(smtpServer, smtpPort, smtpSSL, smtpAuthentication);

Transport transport = session.getTransport();
transport.connect(smtpServer, smtpUser, smtpPassword);

try {
    MimeMessage message = EmailUtilities.createMessage(Main.APPLICATION_NAME + " " + Main.APPLICATION_VERSION, session, to, from, subject, body, attachmentName, attachment, "application/pdf");
    transport.sendMessage(message, message.getAllRecipients());
} catch (Exception e) {
    throw new Exception(e);
} finally {
    transport.close();
}


public static Session createSession(String smtpServer, Integer smtpPort, boolean ssl, boolean authenticate) {
    String protocol = "smtp"; //$NON-NLS-1$
    if (ssl) {
    protocol = "smtps"; //$NON-NLS-1$
    }

    // -- Attaching to default Session, or we could start a new one --
    Properties props = new Properties();
    props.put("mail.transport.protocol", protocol);
    props.put("mail." + protocol + ".host", smtpServer);
    props.put("mail." + protocol + ".port", smtpPort.toString());
    props.put("mail." + protocol + ".starttls.enable", Boolean.TRUE.toString());

    if (authenticate) {
        props.put("mail." + protocol + ".auth", Boolean.TRUE.toString());
    }

    Session session = Session.getDefaultInstance(props, null);
    session.setDebug(true);

    return session;
}