This is a tricky question, as the answer lies solely with the segment of the population you are trying to reach. It will differ depending on who you are trying to target with your email campaigns. The best solution is to do client research, using both the tools available online and a good deal of common sense. If you are trying to target 20-25 year old people, a good time would not be 6-7 p.m. on a Monday to Friday night as they are most likely to be engaged in other social activities.