What makes a good email subject line according to the tester?
The tester scores three main factors. Length should fall between 30 and 50 characters so it does not get truncated on mobile. Spam-trigger words such as free, guarantee, winner and urgent are penalized because they push messages toward filters. ALL CAPS and excessive punctuation also lower the score. A strong subject is concise, specific, written in sentence case and uses curiosity or a clear value statement rather than hype.
Why does subject length matter for open rates?
Most mobile clients display only the first 30 to 40 characters of a subject before truncating with an ellipsis, and over 60 percent of email is now opened on mobile. If your call to action sits at character 65, half your audience never sees it. Shorter subjects also feel less like marketing and more like a personal note, which boosts open rates. Aim for under 50 characters and place the most important word first.
Which spam trigger words should I avoid in subject lines?
Classic offenders include free, cash, winner, guarantee, congratulations, act now, limited time, click here, double your, risk-free, and 100 percent. Excessive currency symbols, exclamation points and ALL CAPS also trigger filters. Modern Bayesian filters look at combinations rather than single words, so one trigger word is usually fine. The tester flags risky words so you can rephrase them in a way that conveys the same urgency without raising spam scores.
Does ALL CAPS in a subject really hurt deliverability?
Yes, on two fronts. Spam filters treat heavy capitalization as a shouting signal that correlates with junk mail in their training data, so it adds points to the spam score. Subscribers also perceive ALL CAPS as aggressive and are more likely to mark the message as spam, which then teaches mailbox providers to filter your future messages. Limit caps to one short word for emphasis at most, and never write the entire subject in capitals.
How does the tester compare to A/B testing in my ESP?
The tester is a fast pre-flight check based on widely studied heuristics. It catches obvious problems like spammy phrasing or truncation before you spend a campaign on a weak subject. It does not replace A/B testing, which measures real audience behavior. Use the tester to filter out poor candidates, then A/B test the two or three strongest variants on a small segment to find the winner with your specific list.
Should I use emojis in email subject lines?
Used sparingly, an emoji can boost open rates by drawing the eye in a crowded inbox, especially for promotional or lifestyle brands. Used poorly, they look spammy and some older clients render them as boxes or question marks. The tester does not penalize emojis directly, but you should preview the subject in Gmail, Outlook and Apple Mail before sending. Stick to one emoji at the start or end, and never replace key words with symbols.
