Many good observations above!

Now, one that has not yet been made in this thread:

You can "validate" the form of an email address using a module; to actually "validate" the address itself, you have to check whether it exists.

This is why you'll find some/many sites with a registration option that handle the process like this:

User fills in and submits the form

Site sends a "please confirm" mail to the address supplied (with or without some sort of confirmation string and a reply-to: address used specifically for the purpose).

If the recipient responds (sometimes, within a specified length of time), site processes the reply and then (and only then) adds the new user to the list of valid emails/members/users.