Return False not working inside jQuery.ajax

P.S.: Read "EDITED on 2019-06-29":

I have a webform for updating user information, and when he updates his email a verification is performed via ajax() so that he is warned if the new email address is already in use by another user.

I'm trying to cancel the form submission when the email is in use, but return false; doesn't work.

Any other return false; within if statements are working fine, the problem is only with this one inside the jQuery.ajax() call.

Here's the actual code:

var email = jQuery('#email').val();
jQuery.ajax({
    type : 'GET',
    url : '/ajax/verify-email.php?email=' + email,
    success : function( d ) {
        if( d == '1' ) {
            alert('Another user is using this email');
            jQuery('input[name="email"]').focus();
            return false; // this guy over here is not working!!!!
        }
    }
});

Does anyone have a solution?

EDITED on 2019-06-29

When I asked this question back in 2012 I wasn't aware of Promises in Javascript, neither knew about "$.when" (even though added 3 years later of this question) to handle asynchronous requests alongside $.ajax.

Today, you can handle the same scenario easily, as this:

let email = $('#email').val();
$.when(
  $.ajax({
    type : 'GET',
    url : `/ajax/verify-email.php?email=${email}` 
  })
 .then(function(d) {
   alert('Another user is using this email');
   $('input[name="email"]').focus();
});
// your code continues from here as desired...