Free tool
Subject line tester.
Paste your subject line to check length, spam triggers, and readability.
Length matters
Keep under 60 characters. Mobile inboxes truncate at ~30-40 characters. Front-load the value.
Spam triggers
Avoid: FREE, ACT NOW, !!!, 100%, guaranteed, urgent, limited time, all caps, excessive punctuation.
What works
Personalization, curiosity gaps, specificity, and relevance to the recipient's role or company.