Email Subject Line Tester
Score your email subject lines before you send. Checks spam trigger words, Gmail/Outlook/Apple Mail character limits, mobile preview clipping, and readability. Free, browser-only.
The Email Subject Line Tester scores your subject lines against the same signals that spam filters, inbox algorithms, and real subscribers use to decide whether to open or ignore your email — all in your browser with no account required.
Type your subject line and get an instant overall score out of 100, broken down across five dimensions: spam risk, length, readability, mobile preview, and preheader quality. The spam engine checks against 100+ trigger words and phrases drawn from the SpamAssassin ruleset and Mailchimp's known filter lists, grouped by category — urgency tactics, money promises, aggressive free offers, deceptive phrasing, health claims, and financial schemes. Every flagged phrase is highlighted so you know exactly what to rephrase.
The length and preview engine shows you how your subject line will appear in six major email clients — Gmail Desktop (clips at ~60 chars), Gmail Mobile (~38 chars), Apple Mail (~55 chars), Outlook Desktop (~60 chars), Outlook Mobile (~33 chars), and Yahoo Mail (~55 chars). Rows that will clip are highlighted in amber so you can decide whether to shorten or accept the truncation. The inbox preview cards simulate the actual sender name, subject, and preheader layout so you can visualise the full inbox row before you send.
The readability dimension scores your word count (sweet spot is 4–8 words), question usage (questions drive curiosity and boost open rates), emoji count (1–2 can improve inbox visibility; 3+ reads as spam), personalisation tokens like {First Name}, and excessive punctuation patterns. The preheader dimension flags the most common mistake — either no preheader at all (leaving email clients to pull random body copy) or a preheader that simply repeats the subject line, wasting the second most-visible line of inbox real estate.
Every suggestion is actionable and specific to your input: which trigger words to remove, how many characters to cut, and what structural changes will move your score up. Use the "Copy Report" button to export a plain-text summary for your team or client review.
Frequently Asked Questions
- What spam trigger words does this tool check against?
The tool checks against 100+ phrases drawn from the SpamAssassin ruleset and Mailchimp\
- How many characters can my subject line be before Gmail clips it?
Gmail Desktop clips subject lines at approximately 60 characters. Gmail Mobile is much shorter — around 38 characters. The tool previews your subject line at all six major client limits and highlights which clients will clip your current subject so you can decide whether to shorten it.
- What is a preheader and why does it matter?
The preheader (also called preview text) is the short snippet of text that appears after the subject line in most inbox views. It\
- Do emojis help or hurt email open rates?
1–2 well-chosen emojis can make your subject line stand out in a crowded inbox and modestly improve open rates, particularly on mobile where the colourful icon is more visible. However, 3 or more emojis — especially combined with ALL CAPS or exclamation marks — triggers spam filters and looks unprofessional. The tool scores your emoji usage and flags when you\
- Is this tool accurate for my ESP (Mailchimp, Klaviyo, etc.)?
The spam scoring is based on widely published filter rules and is a strong directional indicator, but every ESP and receiving mail server runs its own version of spam filtering with proprietary weights. This tool gives you a reliable pre-send sanity check — catching obvious red flags, length issues, and structural problems — but the best final test is always a small list send or your ESP\