I love seeing studies like this one from MarketingSherpa which gives quantifiable and significant evidence that the best time to send an email blast is right around 9am.
"An eretailer wondered how much of a difference they could make by testing the time of day to send their blasts. Turns out a lot -- clickthroughs increased 15.63% and revenue jumped, too."
According to the study, clickthrough rates for emails sent at 9am performed more than 15% better than those sent at 4pm. Of course you need to also consider the timezones of the people in your list, but it's still interesting to note the habits of people at work and when they are most receptive to receiving emails.