Why Your Emails Aren't Getting Opened (And How to Fix the Subject Line)

The Subject Line Nobody Read

An email marketer at a mid-size e-commerce brand ran a Black Friday campaign to 45,000 subscribers. Open rate: 6.1%. The previous year's Black Friday email hit 24%.

Same list. Same product. Different subject line.

The one that bombed: "FREE SHIPPING + BONUS - Act Now - Limited Time Offer!!!!". Seven words that triggered spam filters, three exclamation marks, and 62 characters that got cut off on every phone. The entire hook disappeared before anyone could read it. The email inside was actually pretty good. Nobody saw it.

Subject lines are the door. Most marketers spend hours on the email itself and thirty seconds on the subject line. That's the wrong ratio.

What Email Clients Actually Filter

Spam filtering isn't a mystery anymore. Email providers like Gmail, Outlook, and Yahoo run every inbound message through scoring algorithms looking for specific patterns. A subject line with "FREE" in all caps? Strike. "Guaranteed"? Strike. "Act now", "click here", "no obligation"? Each one costs you deliverability points.

Pile enough of them into a single subject and the email never reaches the inbox. Not because your content was bad. Because your subject line flagged the whole thing before anyone had a chance to see it.

The words people reach for instinctively when writing promotional copy are often the exact words that kill deliverability. "Congratulations", "winner", "earn money", "risk-free." These feel like strong copy. They read like spam to filters.

One trigger word usually isn't fatal. Three is. Stack promotional language and you're essentially writing for the junk folder.

Length, Truncation, and the 40-Character Rule

Here's what most guides mention but don't explain clearly: mobile truncation isn't just cosmetic. When your subject line gets cut off on a phone screen, you don't just lose words. You lose context. A subject that reads fine at 58 characters on desktop can deliver a completely different impression at the 40-character mark on a lock screen.

Take this: "How to save 30% on your annual subscription before the price increase"

Desktop inbox: full context, clear value proposition. iPhone lock screen: "How to save 30% on your annual sub..."

That version still works. But flip the structure: "Annual price increase incoming: lock in 30% savings before Friday"

On mobile that becomes: "Annual price increase incoming: lock..."

Now you've led with news that sounds alarming rather than an opportunity. The framing shifts entirely based on where the cut happens.

Front-load your hook. Put the compelling part within the first 35 to 40 characters. Everything after that is bonus context for desktop readers.

How to Use the Tester

The email subject line tester scores any subject line from 0 to 100 across seven checks: character count, mobile preview truncation, spam triggers, capitalization ratio, punctuation, engagement signals, and word count. Paste your subject in. It scores in real time as you type.

The mobile inbox preview at the bottom is what I keep coming back to. It simulates how your subject appears on a phone screen, including exactly where the text cuts off. No more guessing from a character count. You see the truncation directly.

The engagement signals check is worth paying attention to, too. It looks for things that tend to correlate with higher open rates: a personalization token like , a number or statistic, a question, urgency language, an emoji. These aren't guarantees. A subject line that clears the spam check and includes at least one genuine engagement signal consistently outperforms one that's just technically clean.

The way to use this: write three subject line variations, score all three, and pick the one that combines a strong score with the angle you actually believe in. If two subjects score within five points of each other, the one with the stronger hook wins regardless of the number.

A quick note: Mailchimp has a built-in subject line preview that shows desktop versus mobile rendering, but it doesn't flag spam words or score engagement signals. If you're building campaigns there, the tester and Mailchimp's preview complement each other well.

What the Score Actually Tells You

A score above 70 means your subject is technically clean: reasonable length, no obvious spam triggers, no all-caps, no excessive punctuation. That's the floor, not the goal.

Scores in the 80s usually mean you've also got at least one positive engagement signal. That's where most solid-performing subjects land.

Chasing 95-plus often means you've over-optimized. Perfect technical scores can produce subjects that feel sanitized. Sometimes the subject that converts best reads slightly rough because it sounds like a real person wrote it. Use the score to fix problems, not to flatten your voice.

FAQ

How many spam trigger words are too many?

One is usually fine depending on context. Two in the same subject is risky. Three or more significantly increases the chance of landing in spam, even if everything else is clean. The tester flags each trigger individually so you can see exactly which words are costing you.

Does subject line length affect spam filtering?

Not directly. Spam filters care about content, not character count. Long subjects (over 80 characters) correlate with low engagement over time, which can affect sender reputation, but the mechanism is indirect. The main reason to stay under 60 characters is open rates, not deliverability.

Should I use emojis?

One emoji that relates to your content can lift open rates. Two starts to look like promotional noise. The tester treats a single emoji as an engagement booster. Use them when they fit naturally and skip them for B2B outreach where the tone needs to stay professional.

What if my highest-scoring subject feels boring?

Use it as a starting point. Strip the problematic elements, then add back the personality. A subject that scores 85 and sounds like a human wrote it beats a 92 that reads like a press release. The score identifies what to fix, not what to say.

Can I use this for cold email outreach?

Yes, and it's one of the better use cases. Cold email open rates live and die on the subject line because there's no existing relationship carrying your emails into the inbox. The spam trigger check and the mobile truncation preview are both critical for outreach sequences.

Test Before You Send

Open rates don't recover from a bad subject line. The campaign has already hit the inbox by the time you see the numbers.

Run your next subject through the email subject line tester before you send. It takes twenty seconds. Check the mobile preview. Fix the triggers. Compare a few variations and pick the one that holds up.

If you're working on an email with substantial copy and want to check reading level before writing the subject, the readability checker is a useful stop along the way. And if you're counting characters carefully to stay under a specific limit, the character counter tracks it in real time.