Email Subject Line Tester

Score subject lines on five dimensions — length, spam risk, personalization, emotion and urgency. See what to change before you send.

—

—

—

Scoring Breakdown

Findings

What Makes a Subject Line Work

Subject lines are read in under two seconds on mobile. Five factors dominate open rate:

  • Length — 30–50 characters displays fully on iPhone Mail, Gmail mobile and Outlook.
  • Spam words — "FREE", "ACT NOW", "GUARANTEED", "$$$", excessive caps, and multiple exclamation marks trigger spam filters and reader skips.
  • Personalization — merge tags like {FirstName} or {Company} lift open rates 10–20%.
  • Emotion — curiosity, urgency, exclusivity or benefit. The best subject lines trigger one emotion clearly.
  • Urgency — "tonight", "today only", "last chance" — measurable lift in transactional emails, hurtful in newsletters.

The Preview Text Is Half the Battle

Email clients display the first 40–100 characters of the email body as preview text. If you don't control it, the first sentence of your email will. Write both together — the subject sets up the preview, the preview completes the thought.

Common Mistakes

  • ALL CAPS. Instant spam signal. Avoid.
  • Multiple exclamation marks. One is fine; two or more triggers filters.
  • Starting with "Newsletter" or "Update". Zero curiosity. Dead on arrival.
  • Burying the benefit. Put the hook in the first 30 characters — that's what shows on mobile.
  • Ignoring the sender name. Subscribers scan sender first, subject second. Consistency matters more than subject tricks.

Subject Line FAQ

What is the ideal subject line length?

30–50 characters displays fully on the widest range of mobile and desktop email clients. Under 25 feels thin; over 60 truncates.

Do emoji in subject lines help?

Split tests show slight positive effect when used sparingly (1 emoji) and used topically. Over-use reads as spam to some audiences.

Are spam words the same for everyone?

Gmail, Outlook and corporate filters use overlapping but different lists. "FREE", "ACT NOW", "GUARANTEED" and dollar signs trigger most filters. Also avoid excessive exclamation and all-caps.

Does personalization really work?

Yes — but only first-name and meaningful variables. Fake personalization ("Hi John" when they didn't give you the name) hurts.