Hello,
It is not generally a good idea to send an HTML that is somewhat interactive, like your JS (JavaScript) validation, via email. The thing is that, while some mailers do support HTML messages, the way each mailer deals with such emails differs very much from one another.
For instance, my mailer is capable of receiving HTML messages (though I configured it so that I don't receive such messages, like many others do), yet it automatically strips all JS and other scripting codes before it even gets into my inbox. This is only one example, and I thought Hotmail does more or less the same thing, and so forth.
Furthermore, input validation with JS is not reliable at all, and depending on the kind of data you are dealing with, you should stay away from using it or shouldn't count on it too much anyway - instead, use PHP or other server-side scripting language that the users will have no control over.
Now, back to the topic. What I would recommend is simply give a link to an online form on your site, intead of sending it in the email. I understand this is not really what you wanted to hear, but this way, you can ensure the single point of input (and thus validation) and everyone, no matter what mailer he/she uses, can access your online form eventually.
Also, if you really want (or need) to send out HTML messages, consider using multipart messages for those who have mailers with HTML message disabled. Here's a good example:
http://www.zend.com/zend/trick/html-email.php
HTH.