1. What a subject line has to do

A subject line has one job: earn the open. Not the click, not the sale — just the open. That means it has to do two things at once: stand out in a crowded inbox, and tell the reader honestly what's inside so the open feels worth it. Mailchimp's advice on subject-line best practices sums it up well: write subject lines that tell rather than sell — give people a clear reason to look, not a sales pitch.

2. Get the length right

Most inboxes cut subject lines off somewhere around 50–60 characters, and a large share of email is read on phones, where the cut comes sooner. Aim to get your point across in roughly 40 characters or fewer, and front-load the most important words so they survive truncation. Short isn't just about fitting — punchy, specific subject lines tend to outperform long, vague ones anyway.

3. Subject line formats that work

When you're stuck, these reliably earn opens:

  • Benefit: "Save 3 hours on your next campaign."
  • Curiosity: "The mistake costing you opens."
  • Specific number: "5 subject lines we tested last month."
  • Question: "Is your list actually growing?"
  • Personal / direct: "Quick favour, [first name]?"
  • Urgency (used honestly): "Last day for the free toolkit."

Personalization — a name, a location, a past action — consistently lifts opens, but only when it's relevant. A name slapped on a generic blast doesn't fool anyone.

4. The truth about spam triggers

The old advice to avoid a magic list of "spam words" is largely a myth now. Modern filters judge sender reputation, engagement, and authentication far more than individual words. As Mailchimp explains in its overview of what actually triggers spam filters, the word "free" won't sink you — but bad habits will. What genuinely hurts you:

  • Excessive punctuation — keep it to three marks or fewer; stacked exclamation points lower inbox placement.
  • ALL CAPS and gimmicky symbols.
  • Misleading subjects that don't match the email — this also violates the law (more below).
  • Low engagement — if people don't open your emails, filters notice and route you to spam.
Quick win Read your subject line and ask: "Does the email actually deliver this?" Honesty isn't just ethical — misleading subject lines are explicitly banned under the U.S. CAN-SPAM Act, with steep per-email penalties.

5. Don't forget the preview text

The preview (or preheader) text — the snippet shown next to or under the subject line — is prime real estate most senders waste. Left blank, it pulls in whatever's at the top of your email ("View in browser…"). Used deliberately, it extends the subject line and adds a second reason to open. Treat subject line and preview as a one-two punch: the subject hooks, the preview reinforces.

6. Test before you send

You can't reliably predict the winner, so don't guess:

  • Write several options per send, in different angles.
  • A/B test where your tool allows — even a small split reveals real preferences.
  • Check length, punctuation, and clarity before sending, so an avoidable mistake doesn't cost you the open.

Scoring a subject line for length, spam-ish formatting, urgency and clarity takes seconds with a tester — far cheaper than learning from a flat open rate after the fact.

Test your subject lines before you send

Score your subject lines for length, spam triggers, urgency and clarity before you hit send. Free, no signup.

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A great subject line gets the open — then the benchmarks tell you if it worked. See email open & click rate benchmarks, and the same headline skills carry over to headlines that convert.