I put a widget on both sides. But if you just click the subscribe button (without entering an email), you will get the error popup I’m talking about.
It probably should also look for the period also, and not just the @ symbol.
Then if you put in a proper email, and no name, then you get a different popup, which appears at the top of the page in the center.
Right now I do use a different newsletter widget, but it doesn’t incorporate the user login subscribe button on the wordpress login page.
But if you go to http://personalstylist.ca/ on the right I have the one I currently use. If your email is wrong, it just pops up text in the same spot, then has a simple back button to go back. But I really do like your popup better, which means they don’t have to retype their email.
Hope this helps.