I realize this kind of email subject title will have higher open rate:
1. Short and simple
2. Don't uppercase every single words
3. Don't capitalize each words
4. Don't use spam words like Free, Guarantee, Save $ and so on
5. Curiosity title always work well, but don't over hype
6. Questions or messages that require a respond
7. Have the right mindset like email to you buddy or recommand something good to your friends.
Do you have any add on? Or is these good enough?