javamail - 使用SMTP(非ＳＳＬ）寄出信件時一直收到javax.net.ssl.SSLHandshakeException...

參考：Java Mail: SSLHandshakeException when sending email on port 25 without SSL


需求：
  最近一個小工具需使用到ＪＡＶＡＭＡＩＬ，奇怪的是我明明使用ＳＭＴＰ（非ＳＳＬ）寄信，卻一直報ＳＳＬ的錯誤：

javax.mail.MessagingException: Can't send command to SMTP host;
  nested exception is:
    javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
    at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1420)
    at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1408)
    at com.sun.mail.smtp.SMTPTransport.ehlo(SMTPTransport.java:847)
    at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:384)
    at javax.mail.Service.connect(Service.java:275)
    at javax.mail.Service.connect(Service.java:156)

查了半天才發現，只要關閉下列的Properties設定參數即可：

props.put("mail.smtp.starttls.enable", "true");

（把上一行取消就可）