javax.mail.NoSuchProviderException: Invalid protocol :null

Hi,
I know this is discussed a lot of times in the forum. my case is a bit different, any sort of help is appreciated.

I use activation.jar and mail.jar for sending emails for my application. I am not getting emails in certain cases. The same code is used for sending emails for all the application interfaces. Only in certain cases, i get the following error,

javax.mail.NoSuchProviderException: Invalid protocol :null

This is the code i use,

Properties sessionConfig = new Properties();
sessionConfig.put("mail.transport.protocol",
"smtp");
System.out.println("smtphost "+smtpHost);
sessionConfig.put("mail.smtp.host",
smtpHost);
sessionConfig.put("from",
"from");
sessionConfig.put("body",
"No body for this message.");
sessionConfig.put("subject",
"No subject for this message.");
_session = Session.getDefaultInstance(sessionConfig, null);
transport = _session.getTransport();
transport.connect();
transport.addConnectionListener(new ConnectionEventListener());
transport.addTransportListener(new TransportEventListener());
MimeMessage transportMessage=getMessage(message);
transport.sendMessage(transportMessage,
transportMessage.getAllRecipients());

this happens only in certain cases. And the protocol is set properly. the classpath has only these two jars.