You are competing for their attention. In many cases, the subject is the most important part of your message. In a way, it is your advertisement. Your subject is the knock at the door, asking the reader to click and read what you have to say. Never leave your subject blank.
Most people delete messages with blank subjects automatically.
Your subject should clearly explain the contents and importance of your message in only three or four words. It will work only once. Again, do not overdo this. Paragraphs Make your paragraphs short. Look at junk mail.