According to Dan Zarrella, most weekday email gets read first thing in the morning.
His research indicates that email messages with the highest click-through rate are sent between 6:00AM and 7:00AM EST.
Other valuable insights:
Limit your subject lines to no more than 50 characters;
Wednesday distribution typically offers the highest weekday clickthroughs, averaging about five percent. Weekends are better at around nine percent; and
Cap your email frequency to no more than four per month.