Using a message iframein an email message is not supported. For more information, see Link. It’s also the wrong way to embed videos on YouTube. If you go to YouTube and share the video by email with yourself, you can check the html content of this email to find out how to embed the video correctly.
A static clickable image that redirects the user to YouTube.
<a href="http://www.youtube.com/watch?v=Bk_6r-b3kqU&feature=em-share_video_user">
style="text-decoration:none;display:block"
class="nonplayable"
target="_blank">
<img src="http://i3.ytimg.com/vi/Bk_6r-b3kqU/mqdefault.jpg" height="274" width="498">
< /img></a>
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<embed width="640" height="385" base="https://www.youtube.com/v/" wmode="opaque" id="swfContainer0" type="application/x-shockwave-flash" src="https://www.youtube.com/v/Bk_6r-b3kqU?border=0&autoplay=1&client=ytapi-google-gmail&version=3&start=0">
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