使用@Async注解不生效啊

	@Async	public?void?sendHTMLMail(String?to,?String?subject,?String?content)?throws?MessagingException?{	
		MimeMessage?mimeMessage?=?mailSender.createMimeMessage();		
		MimeMessageHelper?mimeMessageHelper?=?new?MimeMessageHelper(mimeMessage,?true);		
		mimeMessageHelper.setTo(to);		
		mimeMessageHelper.setSubject(subject);		
		mimeMessageHelper.setText(content,?true);		
		mimeMessageHelper.setFrom(from);		
		mailSender.send(mimeMessage);	
		}

但是如果沒有發送郵件的話，異步是可以生效的

	@Async	public?void?sendHTMLMail(String?to,?String?subject,?String?content)?throws?MessagingException?{	
		for?(int?i?=?0;?i?<?20;?i++)?{	
		System.out.println("已經發送?。?！");		
		}	
	}

求解如何實現異步發送郵件？