Question 1

How long should an email subject line be?

Accepted Answer

30-50 characters or 4-7 words. Mobile email clients truncate subject lines at roughly 35 characters. Desktop clients like Gmail show about 60 characters and Outlook around 55. If your subject line is over 50 characters, the most important words need to be front-loaded so they survive truncation.

Question 2

Do emojis improve email open rates?

Accepted Answer

Yes, by 3-5% on average in platform A/B tests (Experian, Campaign Monitor). But overuse triggers spam filters — one emoji maximum. Placement matters too: emojis at the beginning of a subject line draw more attention in crowded inboxes but can look spammy if combined with promotional language.

Question 3

What words trigger spam filters?

Accepted Answer

Common triggers include "free", "guaranteed", "act now", "limited time", "click here", "buy now", "no cost", "risk free", "winner", "congratulations", "urgent", and "order now". ALL CAPS and excessive punctuation (!!!) also flag filters. Platform-specific: Mailchimp is stricter on promotional language than MailerLite, and Klaviyo's ecommerce audience is more tolerant of discount-related terms.

Question 4

Should I use personalization in subject lines?

Accepted Answer

Yes. Merge tags like {first_name} increase open rates 10-14% higher open rates (Campaign Monitor, ActiveCampaign A/B data). But do not fake personalization — "Hey friend" without an actual merge tag feels dishonest and erodes trust. Make sure your subscriber data is clean: a subject line that reads "Hi {first_name}" because the merge tag failed is worse than no personalization at all.

Question 5

What is a good email subject line score?

Accepted Answer

On this tool, 70+ is good and 80+ is excellent. Below 50 means multiple criteria need fixing — typically a combination of length issues, spam triggers, and missing personalization. The score is based on 12 criteria weighted equally, so improving any single low-scoring criterion can lift your overall score meaningfully.

Question 6

Do questions work better than statements in subject lines?

Accepted Answer

Questions see 10-15% higher open rates in A/B tests (Campaign Monitor, Klaviyo). They create curiosity gaps — the reader opens the email to find the answer. Questions that start with "How", "What", "Why", or "Did you" perform particularly well because they imply actionable content inside the email.

Question 7

How does this tool score subject lines?

Accepted Answer

12 criteria, each scored 0-10: character length, word count, spam trigger words, personalization, urgency, question format, number usage, emoji presence, readability (Flesch-Kincaid approximation), preview text opportunity, mobile truncation risk, and capitalization. The raw total is normalized to a 0-100 scale. Each criterion includes specific feedback explaining what to improve.

Question 8

Why does my subject line score differently on different platforms?

Accepted Answer

Each ESP has different spam filters, character rendering, and user demographics. A promotional subject line with discount language may perform well on Klaviyo (ecommerce audience expects promotions) but poorly on MailerLite (stricter spam filtering and approval process). This tool provides platform-specific advice when you select your ESP.

Question 9

What's the best subject line format for ecommerce?

Accepted Answer

Include the product name or discount amount. Numbers outperform vague claims: "25% off running shoes" beats "Amazing sale inside" in A/B tests. For Klaviyo users specifically, flow subject lines (abandoned cart, post-purchase) average 50%+ open rates vs 31% for campaigns — so the format matters less for automated flows where timing does the heavy lifting.

Question 10

Can I test multiple subject lines at once?

Accepted Answer

Use A/B testing in your ESP to test multiple subject lines with live audiences. This tool scores individual lines — test your top 2-3 candidates here and send the highest-scoring one as your primary variant in your ESP's A/B test.

Signal	What the tool checks
Length	Character count, word count, and likely mobile truncation
Clarity	Readability, capitalization, and whether the main idea appears early
Pattern	Questions, numbers, personalization tokens, urgency, and emoji use
Risk flags	Common promotional phrases and excessive punctuation

Free email subject line tester

How the score is calculated