Usually the Message-ID header from a specific sender that uses a yahoo email account is like this:
Message-ID: <1415953854.62001.YahooMailNeo@web172504.mail.ir2.yahoo.com>
And comes along with the X-Mailer header
X-Mailer: YahooMailWebService/0.8.203.733
But sometimes the email headers of the same sender may include a different Message-ID header:
Message-ID: <763084588.2169028.1420389380369.JavaMail.yahoo@jws11101g.mail.ir2.yahoo.com>
In which cases there is no X-Mailer header.
Here it says that the Message-ID header of the first case (...YahooMailNeo...) indicates it was sent via Yahoo Webmail from Chrome.
So does anybody know which interface does JavaMail.yahoo of the second case indicate?