Grade your subject line on length, tone, and spam triggers — instantly.
Type a subject. We score it on the six dimensions mailbox providers actually use — length sweet-spot, caps ratio, emoji density, punctuation, presence of power-words, and presence of spam triggers. Instant A–F grade + per-rule feedback.
Type a subject on the left to grade it.
Mailbox providers use the subject as one of many features fed into their inbox-vs-spam classifier. They also expose snippet length to the user as the primary "should I open this" cue — Gmail truncates around 60 chars on desktop, ~30 on mobile.
The sweet spot from large-scale studies (Mailchimp, Litmus, Campaign Monitor) is 4–7 words at 30–60 characters. Shorter than that lacks context; longer gets truncated. Subjects starting with the recipient's name or a number outperform average open rates by 10–20%.
Avoid ALL-CAPS, multiple exclamation marks, more than one emoji, and any of the obvious spam triggers ("FREE", "URGENT", "ACT NOW"). One emoji at the start can help open-rates in B2C; three+ tank both opens and inbox placement.
Personal tone beats marketing tone for cold outreach. "Quick question about your hiring plans" outperforms "🔥 EXCLUSIVE: Hiring tools for Q4!" by a wide margin in every test we've seen.
Roughly 30–60 characters / 4–7 words. Gmail truncates around 60 chars on desktop and ~30 on mobile, so any signal past that gets cut.
Zero or one emoji is fine and can lift open rates in B2C. Two or more starts to read as spam to filters; three+ is a strong negative signal.
ALL CAPS in the subject is one of the strongest historical spam signals — Bayesian filters have been trained on it since 2003 and the weight is still high.
Subjects with numbers ("3 tips", "the 2026 outlook", "$50 off") out-open subjects without by a measurable margin (Mailchimp, ~15%). Specificity feels concrete and credible.
No. The subject is one signal among many — sender reputation, auth posture, content of the body, list hygiene all matter. A strong subject won't save bad fundamentals.
Subject scoring is a heuristic. The real test is whether the message lands in the inbox or spam — at every major ESP. SieveGuard's Inbox Placement add-on sends to real seed accounts at Gmail, Outlook, Yahoo, ProtonMail, GMX and reports per-ESP.