I have an older BIS service BB 9810 and It still works on the ATT net. I would suggest checking the phone for email passwords and various setup requirements. It may be possible the email setup was corrupted by another App.
As for the setup, here Is the web link
https://www.att.com/devicehowto/tuto...el=Bold9700v60
As for the OS level, I do not think ATT requires any level. The basic operation including Email should work ok. the only issue I have had is when Apps upgrade and require newer OS level to function. I have had that problem on my current BB with the Android level.
Also I checked with ATT and they also said it should work.