I'm not sure I understand what you mean by "direct video," then. If a site, any video-hosting site, is compatible with embed.ly then it'll propagate the thumbnail with a play button in the video block. Otherwise, if the site you're wanting to use isn't compatible with embed.ly, then it's a moot point. In that regard, you'd be using an alternative linking source anyway. You don't have to use YouTube if you don't want to.
Videos cannot be embedded directly into emails for both file size allowances, and to satiate more stringent security algorithms for the larger email programs.
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