Best Practices for Subject Lines:
Keep it short and sweet: Subject lines with fewer than 9 words tend to work best.
Customization and personalization: Use personalization, like a first name, to make the email feel more personal.
Limit punctuation: Too many punctuation marks can make the email look like spam and get flagged as such.
Avoid the usage of “salesy” terminology
Question-based subject lines have been shown to generate interest.
Test, test, test! Try different types of subject lines and then compare open rates. Open rates can reflect how compelling a subject line is.