Email Subject Line Tester
See how your subject line renders across inboxes, check for spam signals, and get actionable recommendations.
We'll score your subject line for deliverability signals and show you exactly how it looks across Gmail, iPhone, Outlook, and more.
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Score Breakdown
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Why Email Subject Lines Matter
Your subject line is the first thing recipients see. It determines whether your email gets opened, ignored, or marked as spam. But subject lines alone don't determine deliverability. Spam filters look at dozens of signals simultaneously: your domain reputation, authentication records (SPF, DKIM, DMARC), recipient engagement history, sending patterns, and content. The subject line is one piece of a much larger puzzle.
That said, subject lines do carry weight. A subject packed with ALL CAPS, multiple exclamation marks, and known spam phrases adds to the overall "spamminess" score that filters calculate. When combined with a new domain, low engagement, or missing authentication, a bad subject line can tip the scales.
What Makes a Good Subject Line?
There is no single formula. The best subject lines match your brand voice and your audience's expectations. A B2B SaaS company and a streetwear brand will have completely different approaches, and both can land in the inbox just fine.
General principles that help:
Be specific. "Your invoice for January" outperforms "Important document inside" because it sets clear expectations. Vague subject lines feel like spam even when they aren't.
Front-load the important words. Mobile devices show 30-40 characters. Put the key information first so it's visible even when truncated.
Match the content. If your subject promises a discount but the email is a newsletter, recipients learn to distrust your subjects. Misleading subjects hurt long-term engagement.
Test with your audience. What works for one list may not work for another. A/B test subjects regularly and track open rates over time, not just for individual sends.
Subject Line Length Across Email Clients
Different email clients display different amounts of your subject line. Here's what you can expect:
Gmail Desktop: Shows approximately 70-77 characters of the subject line, followed by a preview of the email body.
Gmail Mobile (Android): Shows 33-43 characters depending on the device width and whether the sender name is displayed.
Apple Mail (iPhone): Shows 30-33 characters in the notification banner and inbox list view. Lock screen notifications show even less.
Outlook Desktop: Shows about 73 characters in the default view, but this varies with the reading pane layout.
Yahoo Mail: Shows about 46 characters on mobile and 70+ on desktop.
The takeaway: if your most important information is in the first 30 characters, it will be visible everywhere.
Words That Trigger Spam Filters (With Context)
You'll find lists of "spam trigger words" all over the internet. Most of them are outdated or misleading. Modern spam filters don't block emails just because they contain the word "free." If they did, every Black Friday promotional email would land in spam.
What actually matters is the combination of signals. A subject line that says "FREE!!! ACT NOW!!!" from a brand new domain with no DKIM authentication sending to a purchased list will absolutely hit spam. But "Free shipping on orders over $50" from a reputable sender with good engagement will land in the inbox every time.
We flag certain words not because they're guaranteed spam triggers, but because they contribute to cumulative risk. If you're already struggling with deliverability, cleaning up your subject lines is one of many steps that can help.
Frequently Asked Questions
Will a "spam word" in my subject line send me to spam?
Almost certainly not on its own. Modern spam filters (Gmail, Outlook, Yahoo) use machine learning that considers hundreds of signals: sender reputation, authentication, engagement rates, list quality, content patterns, and more. A single word like "free" or "sale" in your subject line is a tiny signal in a massive equation. If your sending reputation is strong and your recipients engage with your emails, you can use almost any word safely.
What is the best subject line length?
There's no universal "best" length. Studies suggest 30-50 characters perform well on average, but this varies hugely by industry and audience. Some of the highest-performing emails use 3-word subjects; others use 80+ characters. The key is that your most important information appears in the first 30 characters so it's visible on mobile. Beyond that, test what works for your specific audience.
Are emojis in subject lines safe?
Yes, emojis are generally safe for deliverability. They don't trigger spam filters. However, be aware that emoji rendering varies across email clients and operating systems, some recipients may see empty squares instead of the intended emoji. One or two emojis can increase visual standout in a crowded inbox. Using many emojis starts to look unprofessional for most B2B contexts, though some B2C brands use them heavily and successfully.
Does ALL CAPS actually hurt deliverability?
An entire subject line in ALL CAPS is a spam signal, yes, but a minor one. It's more of a readability and perception issue: recipients associate ALL CAPS with shouting and spam, which can reduce open rates and increase spam complaints. A single word in caps for emphasis (like "NEW: Product Update") is perfectly fine and common practice.
Does this tool send test emails?
No. This tool analyzes your subject line text entirely in your browser. It checks for common spam signal patterns, calculates length across email clients, and provides visual previews. No emails are sent and nothing is uploaded to our servers. Your subject line is only stored if you choose to unlock the full report with your email address.