I know it's possible to customize the Subscription Confirmation Page, as well as the Forward to a Friend Page (though to a lesser degree)... Seems like it should be possible to alter the Subscription Error Page (emailAddress.php, I believe) too. I've looked all over the help files and the forum but no luck as far as locating some instructions, so far.
Grateful for any advice you all might have.
It's not actually possible to edit that page - instead we would suggest you do your own email validity checking before submitting the form to Campaign Monitor - that will give you full control over the error page.