This is due to security, for the same reason that you cannot put javascript or anything external than images in an email - it can lead to an overly strong email. (You can put things there, they will not be displayed). Unfortunately, this means a lack of reliable flash support.
In fact, most email readers will not even parse simple tags or CSS due to lack of support, I will actually go back to the tables to make sure that the email looks consistent across all different email clients.
Your best / only option is just a link to the video that I'm afraid of. As mentioned above, Gmail will analyze Youtube links and actually implement them for those who have it enabled.
Dunhamzzz
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