Email subject line tester
Get a score, see exactly how your subject looks in the inbox on a phone and a desktop, and fix what holds it back.
Subject line tips that work
How long should a subject line be?
Phones show roughly the first 30–40 characters, desktops 60–70. Aim for 30–50 characters and put the important words first.
Do emoji help?
One relevant emoji can help a subject stand out in a busy inbox. More than two looks cluttered, and some older inboxes show empty boxes.
Should I use the person’s name?
Personal subjects often get more opens. Always set a fallback for people without a first name, e.g. {{first_name|there}}.
What is preview text?
The grey line shown after the subject in most inboxes. If you leave it empty, inboxes show the first words of your email — often “View in browser”.
Which words trigger spam filters?
Phrases like “100% free”, “act now”, “risk-free”, “winner” and repeated “!!!” or “$$$” are classic signals. Write like a person, not an advert.
What’s the best way to know what works?
Test it. MailNesto can send two subject lines to a slice of your list and automatically send the winner to everyone else.