What Makes a Great Email Subject Line?

Research by Mailchimp and HubSpot shows that subject lines between 41–50 characters and 6–10 words consistently achieve the highest open rates. Short lines get cut off on desktop while very long ones get truncated on mobile. The sweet spot balances curiosity, clarity, and urgency without triggering spam filters.

Common Spam Trigger Words to Avoid

Email spam filters flag phrases like “free money”, “click here”, “guaranteed”, “no risk”, and excessive punctuation like “!!!” or ALL CAPS words. This tester checks your subject line against a list of known spam triggers to keep your emails landing in the inbox.