How to alter the "Request new password" form validation
Nothing special about this form, you can alter it just like any other:

function MYMODULE_form_user_pass_alter(&amp;$form, &amp;$form_state) {
  $form['#validate'][] = 'MYMODULE_user_pass_validate';
}

function MYMODULE_user_pass_validate(&amp;$form, &amp;$form_state) {
  // Your code here
}


Works perfectly fine.