/
Test your email subject lines for spam triggers, deliverability, and open rate potential. Get instant feedback and suggestions.
47% of email recipients open emails based on the subject line alone. A great subject line can dramatically improve your open rates, while a poor one can land you in spam.
30-50 characters for best mobile display. 6-10 words optimal.
Using recipient's name can increase open rates by 26%.
Words like "free", "urgent", "winner" trigger spam filters.
Questions and incomplete thoughts drive opens.
Many marketers unknowingly hurt their deliverability by using subject lines that trigger spam filters. Excessive capitalization, misleading preview text, and overuse of punctuation marks like exclamation points are among the most frequent issues. Email providers such as Gmail and Outlook use sophisticated algorithms to evaluate subject lines before deciding whether to deliver messages to the primary inbox or route them to spam.
Another common pitfall is writing subject lines that are too vague or generic. Lines such as "Quick Update" or "Check This Out" lack the specificity needed to stand out in a crowded inbox. Instead, focus on communicating clear value or a specific benefit. Including a number, a deadline, or a relevant detail gives recipients a concrete reason to open your email. Testing multiple variations through A/B testing is one of the most reliable ways to learn what resonates with your particular audience over time.
The Email Subject Line Tester analyzes your subject lines against known deliverability factors that influence whether your emails land in the inbox or the spam folder. It evaluates spam trigger words, character length, capitalization patterns, punctuation usage, and emoji presence to give you an overall score with actionable feedback. Whether you are running a product launch campaign, a weekly newsletter, or a transactional drip sequence, testing subject lines before you hit send can dramatically improve your open rates and sender reputation over time.
Type or paste your email subject line into the input field and the tool instantly returns a deliverability score along with a detailed breakdown. You will see whether your subject line contains common spam trigger words such as "free," "act now," or "limited time." The character count indicator helps you stay within the recommended 30 to 60 character range so your subject line displays fully on both desktop and mobile email clients. Try multiple variations of your subject line and compare scores to find the version most likely to perform well in real inboxes.
Subject lines are the single biggest factor in whether someone opens your email. Research consistently shows that nearly half of recipients decide to open an email based on the subject line alone, and a poorly crafted one can trigger spam filters before your message even reaches the inbox. Email service providers like Gmail and Outlook use sophisticated algorithms to detect spammy patterns, and repeatedly landing in junk folders damages your domain reputation. By testing your subject lines with this tool before every send, you catch potential deliverability issues early and build a consistent track record of high open rates that strengthens your sender score over time.