It's best to write an email's subject line after you're done with the body, and try to make it actionable. But if you want to make sure your email is being clicked, offer either utility or curiosity in the subject, suggests a study by researchers at Carnegie Mellon University.
What do they mean by utility and curiosity?
Utility: These subject lines indicate that the email is directly relevant to someone's work.
For example: "Travel instructions for Tuesday."
Curiosity: Anything that piques people's interest and makes them curious to open the mail.
For example: "Hey." Seriously. In Barack Obama's 2012 campaign, the email that got opened the most was the one with that subject line.
So when you are crafting an email, make sure the subject line says something that the recipient can use in his or her work, or provides just enough information to make them curious enough to see what's inside.