I used to make sun tea in the 80's; however, stopped when I discovered that it harbors bacteria:
The 130°F or so that the water reaches is an ideal temperature for harboring (and growing) the bacteria commonly found in tap water. Without a boiling session to kill off this bacteria, there is the chance that they will grow in the sun tea. The caffeine in black tea does inhibit bacterial growth somewhat. (Herbal tea should never be used to make sun tea.)