Subject lines under 40 characters win

Long subject lines get cut off on mobile. Most of your list reads email on their phone. So everything after about 40 characters is invisible.

The subject lines that perform best say one specific thing. Not a summary of the email. Not a teaser with an ellipsis. One clear reason to open.

"Your cart is waiting" works. "Hey! We noticed you left some items in your cart and wanted to remind you that they're still available with free shipping!" doesn't. The first one fits on a lock screen. The second one gets truncated to "Hey! We noticed you left some items in y..."

Numbers help. "3 tests for this week" beats "Some testing ideas for you." Specificity creates curiosity. Vagueness creates indifference.

Avoid all caps, excessive punctuation, and words like "free" or "urgent" unless you want the spam filter to see your email before your reader does.