Personally, I like to include at least 3 metatags in my emails, to save from any potential future headaches or other pains.

    <meta name="format-detection" content="telephone=no"> 
    <meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0; user-scalable=no;">
    <meta http-equiv="X-UA-Compatible" content="IE=9; IE=8; IE=7; IE=EDGE" />

My preferred viewport tag is a little more verbose compared to the ones in most boilerplates, but I find that it also solves the most headaches for me. Also, I like including the format-detection tag, since even random number strings can, at times, be considered phone numbers and are turned into links on mobile devices.

Finally, the IE Edge tag. This one has a funny story as to why I always include it. I had a boss that always insisted on using the 'compatability mode' button on IE. She actually didn't know what the button did, or why. I started including the EDGE metatag, which strips the compatability mode button from even showing up, and it has saved me a LOT of headaches. Also, since it's been announced that including IE Edge also makes responsive work with Windows Phone, I consider it a win-win. What are your thoughts, guys?