https://www.tutorialspoint.com/javamail_api/javamail_api_smtp_servers.htm
https://www.mkyong.com/java/javamail-api-sending-email-via-gmail-smtp-example/
http://outofmemory.cn/code-snippet/2423/JavaMail-usage-SMTP-protocol-send-dianziyoujian
https://stackoverflow.com/questions/8771167/how-to-change-javamail-port