Article

The One Thing You Should Never Do In An Email Subject (Based on Data)

Brendan G. · Boomerang Free Beginner

This is the actual data behind one of the rules founders lean on: Boomerang analyzed over 300,000 emails and found all-caps subject lines get replies roughly 30 percent less often, partly because spam filters flag them. If you want proof rather than just a rule to trust, this is the two-minute read that gives it to you.

• All-caps subject lines get replies about 30 percent less often than normal-case ones (34.6% vs 50.0% in this dataset).
• Spam detection systems specifically flag all-caps subjects, hurting deliverability before a human ever sees it.
• Urgency-signaling tricks like this often backfire against the very goal they are meant to serve.