Free tool
Subject line tester
Type a subject line and see how it looks on a phone, plus what might hold it back: length, spammy words, capitals and punctuation.
0 characters · 0 words
Not sure which subject wins? Test two.
SendDuo sends two subject lines to a small group, then the winner to everyone else, automatically. Free for up to 2,000 emails a month.
Questions, answered
How long should an email subject line be?
Under 50 characters is a good rule. Phones show about 40, so put the most important words first.
Which words trigger spam filters?
Pushy phrases like "act now", "risk free" or "you have been selected", lots of capitals and strings of ! or $ are classic signs. Modern filters look at much more than words, but these still put readers off.
Do emoji help?
One emoji can help an email stand out; several start to look busy, and a few inboxes show them as empty boxes.
Is my subject line saved?
No. The tester runs in your browser; what you type is never sent anywhere.
More free tools: Email domain checker (SPF, DKIM, DMARC).