How should I test and optimize email campaigns?

Severinghaus suggests taking a scientific approach to optimizing your email campaigns:
Ask a question – how do I improve engagement?

Formulate a hypothesis – will sending emails earlier in the day result in higher levels of engagement?
Test the hypothesis by sending one set of emails in the morning and one set in the afternoon.
Analyze and document the results to see if your hypothesis is correct. Did the emails sent in the morning have higher levels of engagement than the emails sent in the afternoon?
Learn from the results to improve future campaigns. If the performance was better during a certain time, send future emails at the time of day that performed better.
Then repeat.