How long should an email subject line be?

Short enough that its point survives being cut. Phones show about the first 40 characters of a subject and desktop inboxes about 70, so put the words that matter first.

Do spam words send my email to spam?

Not on their own. Mailbox providers weigh your domain's authentication, your sending reputation and how recipients react far more than any single word. Pushy wording and shouting still make a message read like an advert, which is why the tester flags them as notes, not failures.

The short line an inbox shows after the subject. Without one, the inbox shows the first text in the body, often a 'View in browser' link. Use it to add what the subject leaves out.

Free tool

Email subject line tester

See how much of a subject line and preheader phones and desktops show, and catch merge tags that did not render, shouting and spammy habits.

Last updated 2026-10-21

Run it with curl

This tool is also a free API: no account, no key, CORS open. OpenAPI description.

curl "https://sendly.now/api/tools/subject-line-tester?subject=Your%20March%20invoice%20is%20ready"

# Markdown instead of JSON:
curl -H "Accept: text/markdown" "https://sendly.now/api/tools/subject-line-tester?subject=Your%20March%20invoice%20is%20ready"

What makes a subject work

The point comes first, because phones cut a subject after about 40 characters.
It says what the message is, so the reader can decide to open it.
The preheader adds what the subject leaves out instead of repeating it.
Nothing in it is a trick: no fake Re:, no false urgency, no capitals for volume.

What this tester does not do

It does not predict whether a message reaches the inbox. Mailbox providers decide that mostly from your domain's authentication and from how recipients treat your mail, not from words. Check the domain side with the domain health check.

Questions people ask

How long should an email subject line be?: Short enough that its point survives being cut. Phones show about the first 40 characters of a subject and desktop inboxes about 70, so put the words that matter first.
Do spam words send my email to spam?: Not on their own. Mailbox providers weigh your domain's authentication, your sending reputation and how recipients react far more than any single word. Pushy wording and shouting still make a message read like an advert, which is why the tester flags them as notes, not failures.
What is a preheader?: The short line an inbox shows after the subject. Without one, the inbox shows the first text in the body, often a 'View in browser' link. Use it to add what the subject leaves out.

Something on this page out of date, or missing the case you are in? Write to support@sendly.now — we answer our own email. The full technical detail lives in the documentation.