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.