MailAPI SMTP sending error: 'Could not convert socket to TLS'

Question

I'm stucked with smtp authentication in a project.
The process works on port 25 with no STARTTLS, but unfortunately our company server uses port 587 and STARTTLS.
It seems that only the half of the authentication process could be successful, as the server accepts EHLO, but when it switches to secure the program fails with the exception:
'Could not convert socket to TLS'
The simplified code (from a sample app) is:
public void SendMail() throws Exception{
        Properties props = new Properties();
        props.setProperty("mail.transport.protocol", "smtp");
        props.setProperty("mail.smtp.host", "smtpserver");
        props.setProperty("mail.smtp.port", "587");
        props.setProperty("mail.smtp.auth", "true");
        props.setProperty("mail.smtp.auth.ntlm.domain", "codomain");
        props.setProperty("mail.smtp.starttls.required", "true");
        props.setProperty("mail.smtp.auth.mechanisms", "LOGIN NTLM");
        Session mailSession = Session.getDefaultInstance(props, null);
        mailSession.setDebug(true);
        Transport transport = mailSession.getTransport();
        MimeMessage message = new MimeMessage(mailSession);
        message.setSubject("My subject");
        message.setFrom(new InternetAddress("[email protected]"));
        message.setContent("<h1>Hello world</h1>", "text/html");
        message.addRecipient(Message.RecipientType.TO,
                            new InternetAddress("[email protected]"));
        transport.connect("smtpserver", 587, "CODOMAIN\\myaccount", "mypass");
        transport.sendMessage(message,
        message.getRecipients(Message.RecipientType.TO));
        transport.close();
   }

Answer 1

I'll copy the full message below:
+2010.06.29. 12:03:10 mailapi_2.Main main+
SEVERE: null
javax.mail.MessagingException: Could not convert socket to TLS;+
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.startTLS(SMTPTransport.java:1652)
at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:534)
at javax.mail.Service.connect(Service.java:291)
at mailapi_2.SimpleMail.SendMail1(SimpleMail.java:59)
at mailapi_2.Main.main(Main.java:23)
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(Alerts.java:174)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1591)
at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:187)
at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:181)
at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1035)
at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:124)
at com.sun.net.ssl.internal.ssl.Handshaker.processLoop(Handshaker.java:516)
at com.sun.net.ssl.internal.ssl.Handshaker.process_record(Handshaker.java:454)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:884)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1096)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1123)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1107)
at com.sun.mail.util.SocketFetcher.configureSSLSocket(SocketFetcher.java:503)
at com.sun.mail.util.SocketFetcher.startTLS(SocketFetcher.java:443)
at com.sun.mail.smtp.SMTPTransport.startTLS(SMTPTransport.java:1647)
+... 4 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(PKIXValidator.java:285)
at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:191)
at sun.security.validator.Validator.validate(Validator.java:218)
at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:126)
at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:209)
Could not convert socket to TLS
at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:249)
at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1014)
+... 14 more+
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:174)
at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:238)
at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:280)
+... 20 more+
And the output panel before the exception:
Sending mail...
DEBUG: setDebug: JavaMail version 1.4.3
+DEBUG: getProvider() returning javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Sun Microsystems, Inc]+
DEBUG SMTP: useEhlo true, useAuth true
DEBUG SMTP: trying to connect to host "smtpserver", port 587, isSSL false
+220 exch.plt.local Microsoft ESMTP MAIL Service ready at Tue, 29 Jun 2010 12:03:08 +0200+
DEBUG SMTP: connected to host "smtpserver", port: 587
EHLO machine.codomain.local
+250-smtpserver.codomain.local Hello [xxx.xxx.xxx.xxx]+
+250-SIZE 15728640+
+250-PIPELINING+
+250-DSN+
+250-ENHANCEDSTATUSCODES+
+250-STARTTLS+
+250-AUTH GSSAPI NTLM+
+250-8BITMIME+
+250-BINARYMIME+
+250 CHUNKING+
DEBUG SMTP: Found extension "SIZE", arg "15728640"
DEBUG SMTP: Found extension "PIPELINING", arg ""
DEBUG SMTP: Found extension "DSN", arg ""
DEBUG SMTP: Found extension "ENHANCEDSTATUSCODES", arg ""
DEBUG SMTP: Found extension "STARTTLS", arg ""
DEBUG SMTP: Found extension "AUTH", arg "GSSAPI NTLM"
DEBUG SMTP: Found extension "8BITMIME", arg ""
DEBUG SMTP: Found extension "BINARYMIME", arg ""
DEBUG SMTP: Found extension "CHUNKING", arg ""
STARTTLS
+220 2.0.0 SMTP server ready+