The rule: Your subject line should clearly reflect the content of the email.
Your email is important and the recipient may want to find it one day in the future. So, don’t let it get lost in the clutter. Clear subject lines let recipients find information quickly without opening multiple threads and spending valuable time reading through them.
Bad subject line: Interesting thing
Good subject line: Interesting video about vegan dog food
Key takeaway: Add a few more words to the subject line so people can easily reference every email you send them (without weeding through inboxes and folders).