Email and blog headlines are the main reason that people continue reading what you write. Far more so than the content of the said email or blog. Given that you only have a few seconds to really grab the person with the headline, you need to make sure that it does the job every time.
One could even look further and apply these ideas to headlines of digital adverts, call-to-actions, website title pages and so on. It all comes down to the psychology behind what engages people.