The YouTube Terms of Service prohibits downloading YouTube videos, so you cannot email any YouTube videos by uploading them as email attachments. You can, however, email a link to a YouTube video by copying its URL from your browser address bar. If you have a Google Account, you can send an email with a link to the video directly from any YouTube video page.
Sending YouTube Video URLs
Launch the YouTube video in any Web browser. Highlight the URL that shows up on the address bar of the Web browser and then press "Ctrl-C" to copy it to your clipboard. Alternately, click the "Share" button beneath the video pane to bring up a shortened video URL; highlight it and then press "Ctrl-C" to copy it. To add the URL to the email, right-click on a vacant area within the message and then select "Paste." After you send the email, the recipient can either click the URL within the email or copy it to the address bar of a Web browser to view the YouTube video. If you have a Google Account, click the "Share" button, click the "Email" tab, sign in to your Google Account and then enter the address of the recipient into the "Enter Email" field. Click "Send" to send out the email.