Email subject lines: what actually gets your emails opened

The short answer: subject lines under 10 words consistently outperform longer ones, emojis have almost no effect on open rates, and most “spam words” don’t actually hurt deliverability the way conventional advice claims. Three words are the exception: “congratulations,” “urgent,” and “make money” showed reliably lower open rates across thousands of campaigns.

That’s based on an analysis of over 3 million campaigns and 14 billion emails sent through Flodesk. The rest of this post breaks down what the data shows, what it means for how you write subject lines, and how to test what works for your specific audience using Flodesk’s built-in A/B testing.

Most email senders spend the majority of their time on the email body: the design, the copy, the offer. The subject line gets whatever’s left over.

That’s a mismatch. A beautifully designed email that nobody opens is just a beautiful file sitting unread. Your subject line is the gatekeeper. Getting it right, even marginally, compounds over every campaign you send.

The good news: subject lines aren’t a mystery. They follow patterns. And once you know what the data says, you can stop guessing.

Before we get into the data, one honest caveat: open rates have been a murkier metric since Apple introduced Mail Privacy Protection (MPP) in 2021, a feature that pre-loads email tracking pixels for Apple Mail users so that some opens get recorded even when a subscriber doesn’t actually open the email.

This affects open rate benchmarks across the industry. It doesn’t make open rates useless. At scale, they’re still the most practical signal available. But it’s worth knowing that the numbers you see aren’t a perfect count of human eyeballs. We’ll point out where this is especially relevant as we go.

We looked at over 3 million campaigns (more than 14 billion emails sent) and categorized subject lines by their structural patterns. Here’s what stood out.

Long subject lines are the most common non-standard pattern in our data: about 28% of campaigns used subject lines with 10 or more words. They’re also consistently the worst performing.

Open rates for emails with subject lines under 10 words outperformed longer ones by roughly 15%. Click rates followed the same pattern. Longer subject lines, across the board, correlated with lower engagement.

The likely reason: clarity. A short subject line makes a specific promise. A long one dilutes it, and often gets cut off in the inbox preview anyway.

The takeaway: aim for 6–9 words as a general target. If your subject line is running long, it’s usually a sign you’re trying to say too much.

Emoji in subject lines is one of the most debated tactics in email marketing. Our data gives a pretty clear answer: it largely doesn’t matter.

Emojis showed no meaningful impact on open rates. Whether a campaign included an emoji or not, performance landed in essentially the same range.

Emoji placement showed similarly flat results. Subject lines with emojis at the start, end, or middle all performed within a narrow range. The one exception: using emojis at both the start and end of a subject line dropped open rates by 43%! Bookending with emojis appears to be a distinct pattern that underperforms.

The takeaway: use emojis if they fit your brand and the email’s tone. Don’t use them as a performance tactic, and don’t avoid them out of fear. The one thing to skip: emoji at both ends.

Subject lines with repeated punctuation (think “Don’t miss this!!” or “Ready?? Let’s go.”) showed slightly higher open rates than standard subject lines, but lower click rates.

That’s a nuanced finding. Repeated punctuation may attract attention in the inbox, but it doesn’t appear to translate into deeper engagement. The reader opens; they may not act.

The takeaway: use punctuation intentionally. One strong punctuation mark lands better than stacking them.

ALL CAPS: not enough data to conclude

We included all-caps subject lines in the analysis. The sample was too small to draw reliable conclusions, so we’re not making any claims here. What we can say: the senders using all-caps subjects are a very small minority (under 0.02% of campaigns), which suggests most people already instinctively avoid it.

Here’s where conventional wisdom really falls apart.

Most “spam words to avoid” lists circulating online were written years ago, based on how spam filters worked then. They include words like “discount,” “limited time,” “free,” “guaranteed,” and dozens of others. The implication is that using these words will tank your deliverability or your open rates.

We tested them. Across Flodesk campaigns, here’s what we found:

Most classic “spam words” performed at or above the platform average, contradicting the conventional advice to avoid them. A few examples, tested across thousands of campaigns each:

These aren’t the deliverability killers they’re made out to be, at least not in terms of open rate performance at the scale we measured.

That said, three words did show a reliable negative signal, with open rates well below the platform average across thousands of campaigns each:

These stood out as words with both meaningful sample sizes and open rates well below the platform baseline. It’s worth noting that correlation isn’t causation. Senders using these words may have other list health or deliverability factors at play. But the pattern is consistent enough to be worth knowing.

The takeaway: don’t let a “spam words” list stop you from using natural, relevant language in your subject lines. The data doesn’t support broad avoidance of common marketing terms. Focus instead on whether your subject line is honest, specific, and relevant to your audience. That’s what earns the open.

Preview text (also called preheader text) is the snippet of copy that appears after your subject line in most inboxes. It’s prime real estate, and most senders aren’t using it strategically.

In our data, about 97% of campaigns used what we’d classify as standard preview text. Repeated punctuation in preview text showed similar open rates to normal preview text but slightly lower click rates, a minor negative effect, but worth avoiding.

The bigger opportunity isn’t what to avoid; it’s what to use it for. Preview text that extends or complements your subject line adds a second hook. Instead of letting it default to “View in browser” or the first line of your email body, write it intentionally.

A few approaches that work:

Preview text is short (typically 40–130 characters depending on the email client), so keep it tight and make it count.

Data tells you what patterns correlate with performance. It doesn’t write your subject lines. Here’s a framework for doing that well.

The most effective subject lines do one of two things: they tell the reader exactly what’s inside (clarity), or they create a specific open loop that the email resolves (curiosity). Trying to do both usually produces a vague, generic subject line that does neither.

Clarity works well for: offers, announcements, content updates, and anything with a clear “what’s in it for me.” Curiosity works well for: storytelling emails, behind-the-scenes content, or anything where the payoff is the experience of reading.

Vague subject lines underperform. “Big announcement” does less work than “We’re launching something new on Friday.” “A few things I’ve been thinking about” does less work than “Why I stopped discounting, and what happened next.”

Specificity creates a promise. The email either keeps it or breaks it. Over time, keeping promises builds the open habit with your audience.

First-name personalization in subject lines can lift open rates, but only when it feels natural rather than mechanical. “Hey [first name], don’t miss this” reads as a template. Using personalization tied to something meaningful (a location, a recent purchase, a segment behavior) is where it earns its keep.

Your subject line sets an expectation. If it’s playful and the email is a hard sell, the disconnect lands badly. Read your subject line alongside the first two sentences of your email before sending. Do they feel like they belong together?

All the patterns above are starting points. What actually works for your audience is something only your data can tell you, and that’s where A/B testing comes in.

Flodesk’s built-in A/B testing lets you send two subject line variations to a portion of your list and automatically delivers the winning subject line to the rest. No manual tracking, no spreadsheets.

How it works:

The winner is determined by open rate. Keep the MPP caveat in mind here: Apple Mail pre-loading can inflate open rate counts for Apple users, which means results won’t be a perfect read. That said, at the volume of a real A/B test, open rate is still the most practical signal you have available.

What to test:

Test one variable at a time. If you change the length, the tone, and add an emoji all at once, you won’t know what drove the result. Start with the element you’re most uncertain about.

Good variables to start with:

For step-by-step setup instructions, see the Flodesk help center guide to A/B testing subject lines.

Running consistent tests and learning from them over time is much easier when your platform is built for it. Flodesk’s subject line A/B testing is built into the send flow. No third-party tools, no manual audience splitting. You set it up once per campaign and let the results do the work.

Beyond testing, Flodesk gives you the design tools to make sure the email that follows your subject line is worth opening: beautiful templates, flexible layouts, and email automations that keep your list engaged between campaigns.

Before you hit send, run through these: