The big problem is that sometime it works fine, sometime it does not send out any email so I can not determine the problem. I use gmail server to send out the email, it has been working for 2 years without a problem.
The SMTP debugging output is shown below:
2016-02-23 02:02:01 Connection: opening to ssl://smtp.gmail.com:465, timeout=300, options=array (
)
2016-02-23 02:02:16 SMTP ERROR: Failed to connect to server: php_network_getaddresses: getaddrinfo failed: Temporary failure in name resolution (0)
2016-02-23 02:02:16 SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting