Calm down and pull your finger off the shift key. ^_^
When entering an e-mail address, be sure you aren't including a space at the beginning or end. This is a common mistake when pasting it from elsewhere, and it won't be accepted if there is a space in the textbox.
If that is not the problem, it would help us to know what e-mail address you were entering. You should also paste the exact message you received.