Short and sweet: Subject lines with fewer than 9 words tend to work best.

Customization and personalization: You can use customization, like first name, in subject lines to make the email feel more personal.

Limit punctuation: Too many punctuation marks can make the email feel, look, and be registered as spam.

Avoid the usage of “salesy” terminology

Questions as subject lines have shown to do well in terms of creating an interest upon the recipient.

Test, test, test! Try different types of subject lines and then compare open rates. Open rates can reflect how compelling a subject line is.