Short and Sweet and To the Point
When it comes to subject headers, less is usually more.
As a general rule of thumb, it’s a good idea to keep your subject headers at 50 characters or less. Too long, and it will just cut off at the end and viewers won't be able to read the entire subject line.
After you've crafted a subject line, look to eliminate as many unnecessary words as possible.
For example, you rarely ever need to include words like "update" or "newsletter". Obviously, if you are emailing a customer, it is to give them some sort of update. And, readers don't really need to know the format of the email in the subject line.
Be Conversational, Personal, and Respectful
Have you ever been talking to someone you've only meant once or twice, and they asked you about a detail of your life from your last conversation?
Bet it made you feel pretty special, didn't it?
Of course it did. Everyone loves it when someone remembers a personal detail about them.
This same concept can be carried over into email subject headers. Of course, using someone's name in a header is already par for the course these days. Going the extra mile by adding some other unique detail, such as the city the person lives in or their occupation, is a great way to catch your readers attention.
Also, keep it conversational by skipping the "Dear Mr./Mrs.". Saying "hi" or "hey" , instead, helps to keep it more relaxed.