First, I don't think you could even classify Outkast as a "pop" act, at least not in the sense that Timberlake and Stefani are "pop" acts. Whether they're successful at it or not, Outkast definitely are trying to be more than just a radio-hit machine.
Second, if you do choose to define Outkast as "pop" music, then I think you'd have to throw REM, U2, the White Stripes, etc. in there too - in other words, everything that's on a major label and that releases singles meant for radio airplay.