Test a subject line
Check your subject, preview text and sender name for spam triggers, shouting and stray characters, and see roughly where three inboxes cut them off.
How it looks in the inbox
Approximate. Where an inbox cuts the line depends on the font, the screen width and the length of the sender name.
Know which subject actually worked
Email Digit A/B tests subject lines and picks the winner by replies, not opens, because a mail app opening a message for its owner says nothing about the subject.
What the tester looks at
The subject, the preview text and the sender name are the three things a reader sees before deciding to open, and the first things a spam filter reads. The tester runs the same subject rules as our spam score checker, then adds checks that only matter in an inbox list.
The score is a heuristic
Each problem found takes points off 100. It is a checklist turned into a number, not a prediction of opens or of inbox placement. A subject with nothing wrong can still be one nobody wants to open.
Personalisation tokens
A token such as {{first_name}} is fine in a template, as long as it has a fallback. Without one, a contact with no first name gets a subject that starts with a comma.
Subject line questions
Does a high score mean more people will open it?
No. The score counts known problems: spam trigger phrases, shouting, stray characters, a fake RE:, a length that gets cut off. It can't tell you whether your readers will care. Test two versions on your own list for that, and judge them by replies and clicks rather than opens, which mail apps now trigger on their own.
How long should a subject line be?
Short enough that the point survives on a phone, which shows roughly 35 to 40 characters. Desktop inboxes show more, around 70, but most mail is read on phones. The exact cut depends on the font, the screen and how long your sender name is.
Why does preview text matter?
It is the grey line after the subject. If you don't set it, the inbox fills it with the first words of your email, which is often a 'view in browser' link or an image description.
Is my subject line stored?
No. It is checked and thrown away. Only if you ask us to email you the result do we keep your address and the subject you tested.