Hi,
Despite this not being about the Messages app I would suggest these things.
In the email just mouse over any links and see if they do in fact come from AT&T
Although Malware and certainly Viruses are possible but not as common as on windows with Viruses still having to break through the "in the wild" ceiling (i.e. they have been created in software labs as proof of concept).
If AT&T is your ISP I would contact them by phone rather than clicking any links and find out if they did send the email.
I am not saying your Mac is not infected but if you are running a Windows portion in either Boot Camp or VMFusion, Parallels or similar software that can allow you to run other OSes you may have an issue.
I searched (Google) for Mac Botnets but everything seems to be dated 2012
This sounds more like a Phishing attempt.
8:24 pm Wednesday; January 1, 2014
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad