1. The bill came on an unusual day of the month.
2. There's an extra space at the top of the email.
3. It says "Dear Customer" instead of "Dear [your name]"
4. It *doesn't* give your account number, and instead, says how much the bill is for.
5. When you hover over a link, it doesn't go to att.com and instead points to somewhere strange.
Of course, if you've read this, they have too, so perhaps next time, they'll get these details right. (Although, as with spelling errors and weird English syntax in Nigerian scam letters, perhaps that's never going to happen.)
At some point, I guess we're going to need to be able to verify the sender of an email. Until then, keep your eyes wide open. Should I say never click a link in an email? Perhaps from an institution, yes. What a drag.
I'm not sure "sophisticated" is the right word. Perhaps you wouldn't have been tricked. But compare this picture with one of your invoices, and then tell me.