Hello! In regards to adding a youtube video to your email, I would like to add an update to this thread.
The best way to add a youtube video to an email is not to use the "insert youtube video" tool in the email editor. Instead, simply place a link to the youtube video.
Very few email inbox services allow embedding a youtube video in the email, but they will all allow a link to a youtube video.
Also, Gmail (the internet's most popular inbox service) will automatically add a preview image of the youtube video, and allow the user to view the youtube video at the bottom of the screen, as long as there is a youtube link in the video.
That's all that is needed, to simply place the youtube link in the email!