By not being a "valid" email address, I think he means that it has to do with formatting, not that the email address actually exists.
A properly formatted email address will still send, it will just bounce back if the recipient doesn't exist. The only time I ever ran into this problem, that was the cause - there was a space or a dot where it wasn't supposed to be.
As far as user entry, it's necessary to have form validation on email addy input fields - if it passes validation for an email address field, then it will work with UE.
