code: private Session getSession(boolean isSingleton) {
  		final MailAccount mailAccount = this.mailAccount;
  		Authenticator authenticator = null;
  		if (mailAccount.isAuth()) {
  			authenticator = new UserPassAuthenticator(mailAccount.getUser(), mailAccount.getPass());
  		}
  
  		return isSingleton ? Session.getDefaultInstance(mailAccount.getSmtpProps(), authenticator) //
  				: Session.getInstance(mailAccount.getSmtpProps(), authenticator);
  	}
docs: 获取默认邮件会话<br>
  如果为全局单例的会话，则全局只允许一个邮件帐号，否则每次发送邮件会新建一个新的会话
  
  @param isSingleton 是否使用单例Session
  @return 邮件会话 {@link Session}
  @since 4.0.2