To set rule for running Application using email, you have to use email client software such as Microsoft Outlook or others.
I am here using Microsoft Outlook 2010 for the Email client software and use email from Gmail.
(to set Gmail using Microsoft Outlook you can see at previous post)
Now if you already have email on Ms. Outlook, you can see the example below how to set rule for running the application using Subject as a trigger in Microsoft Outlook 2010.
(also you can use Ms. Outlook 2003 or 2007, with little different in menu)
Here in example I want to run Notepad when receive the e-mail with subject : run_notepad
(of course you can set different application, different subject, up to you)
• Open Microsoft Outlook 2010
• Go to File tab
• Choose Manage Rules & Alerts
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