Email Subject Line Tester.
Enter your cold outreach or newsletter subject line and click test to check mobile truncation, spam triggers, fake reply flags, and open-rate score.
What this subject tester checks:
Enter your subject line on the left and click Test Subject Line to inspect:
Re:/Fwd: prefixes, obfuscated text, shouting caps, and promo hype.Explore Other Free Diagnostics
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Subject Line Testing FAQs.
Optimize character length, mobile visibility, and deliverability signals before launching your outreach sequence.
What is the ideal length for an email subject line?
Aim for 24 to 45 characters (3 to 7 words). Most mobile email apps truncate subject lines around 41 characters, so put your main hook up front.
Do ALL-CAPS or fake "Re:" prefixes hurt deliverability?
Yes. Both automated ISP spam filters and human recipients penalize uppercase shouting, consecutive punctuation (!!), and deceptive Re: prefixes on cold emails.
Should I include personalization tokens?
Using a recipient's company or first name naturally in the subject line signals a 1-to-1 personal email rather than a mass blast, improving open rates.
Clean your entire email list with zero bounces.
Sortwell checks addresses the way a mail server would — live SMTP handshake, syntax, MX verification, and spamtrap detection.