Better still, disable the account if the reply is not received within, say, 48 hours of the first address change. This would avoid a shut-out period that might be incurred with a legitimate change and also allow users to make a correction if they realise that they've made a mistake with the address change.
>user could not use an e-mail address like email@example.com.
>That is my default e-mail address for 90% of the online forms I fill out.
>I feel sorry for the person who actually gets the mail at that address.
If you want a non-existent e-mail address you can use with a clear conscience try one of the following:
You can replace anyword with (err...) any word. The Internet Assigned Numbers Authority (IANA) has reserved these second level domain names for experimentation and testing so you can use them as much as you like safe in the knowledge that any e-mail sent to them will just vanish into a black hole.