从国外找到的答案

Starting with Magento 1.9.1 the emails are not being sent directly during checkout but instead are being queued. The queue is being processed via your Magento cronjob – please ensure this has been set up and is running correctly.

解决办法一

If your mail system(smtp, zend_mail) works fine; disabling mailQueue may solve your problem.

/app/code/core/Mage/Core/Model/Email/Template.php

Change Line 407

if ($this->hasQueue() && $this->getQueue() instanceof Mage_Core_Model_Email_Queue) {
if (false /\*$this->hasQueue() && $this->getQueue() instanceof Mage_Core_Model_Email_Queue\*/) {
 

解决办法二 开启magento cron jobs