Some subject line sins kill opens instantly.

Mistakes to avoid:

• ALL CAPS: Looks like spam and triggers filters
• Excessive punctuation: "AMAZING DEAL!!!" feels desperate
• Misleading content: Opens mean nothing if trust is lost
• Too long: Gets cut off on mobile (keep under 40 characters)
• Too vague: "Newsletter #47" gives no reason to open
• Spam trigger words: "Free," "Act now," "Limited time" in excess

Preview your subject line on mobile before sending.