Question: What is Transport in JavaMail api?

Answer: This is final part of sending Email, it is an abstract class. Default version of this class can be used by calling static send() method.

Transport.send(message);

Or can create a specific instance from the session for defined protocol.

message.saveChanges();
Transport transport = session.getTransport("smtp");
transport.connect(host, username, password);
transport.sendMessage(message, message.getAllRecipients());
transport.close();