If you have added an Email App and the red exclamation mark appears as in the image below it simply means that there was a mistake in one of the details you’ve entered.
Click “edit” and make sure that you inserted the user name in the correct form.
A common reason is adding the entire URL (wrong: http://twitter.com/johnsmith) rather than only the user name (correct: johnsmith)
Did you insert your username correctly and still see the exclamation mark? Maybe the site of origin (e.g. twitter) is temporarily down. Wait a couple of minutes and it should be ok.