code: protected static void checkCreateSession ()
      {
          if (_defaultSession == null) { // no need to sync
              Properties props = System.getProperties();
              if (props.getProperty("mail.smtp.host") == null) {
                  props.put("mail.smtp.host", "localhost");
              }
              _defaultSession = Session.getDefaultInstance(props, null);
          }
      }
docs: Create our default session if not already created.