I just sent out a bunch of generic mails, only the name has been replaced to match the youtuber. I only wrote a little more personalized mails when I felt the game is appropriate for a particular youtuber. I reached out to small youtubers
(16-10000 subs), only 10%-15% of them replied back. But it is my personal case and the game is not very marketable, just look up "Trap Them" in the link below and decide yourself. Almost none of the youtubers are suitable to present it, none of them will make it beyond the introductory stages. But it is always fun to see their individual take on the game.
I think it is important to note that your mail should still be short, even when it is personal. You have to imagine big youtubers are flooded with mails and they cannot read all of them anyway. So your mail should be catchy and straight to the point. The creator of "INK" on here made a nice sum up on how to get your game noticed. He suggests to include a pic of your favorite animal in your mail, haha. That is not my style, but feel free to check out his vid and see for yourself:
http://forums.tigsource.com/index.php?topic=48968.0