Email subject line keywords: what works and what trips filters
There is no secret banned-word list. What actually correlates with opens, which phrases correlate with spam, and how modern filters decide.
Writing · 4 min read · updated
First, kill the myth
The “500 spam trigger words” lists that circulate are largely folklore from the era when filters were keyword matchers. They have not worked that way for a long time. A modern filter weighs authentication, sending-domain and IP reputation, how people at that provider have treated your mail lately, complaint rate, list quality and content — together.
Writing “free” will not send you to spam. A brand-new domain with no DKIM, mailing a purchased list, writing “FREE!!! ACT NOW”, will — and would have without the word.
So why does the vocabulary still matter? Because the phrases on those lists are the phrases that appear in mail people do not want, and they are the phrases that make readers hit the spam button. The words are a symptom worth listening to, not a cause worth fearing.
Patterns that correlate with opens
- Specificity. “Your March invoice is ready” beats “Important account update”. Vagueness reads as mass mail because it usually is.
- Numbers. Concrete quantities and dates give the eye something to catch and set an expectation the body can meet.
- The reader's own world. The name of their plan, their city, the feature they use. Not a merge-tagged first name — the actual thing they would recognise.
- An honest curiosity gap. Say enough that they want the rest. Withhold something you genuinely deliver two lines later. A gap you do not close is clickbait, and it costs you the next ten sends.
- A real deadline. Time bounds work when they are true. A recurring “last chance” teaches the reader to ignore all of them.
- Plain questions. “Still want these?” does a lot of work for re-engagement — it is honest, short, and easy to answer.
Patterns that hurt — and what to write instead
- ALL CAPS and !!!. Reads as shouting to the reader and as a low-quality signal to the filter. Write sentence case; earn the emphasis with the words.
- Fake “Re:” and “Fwd:”. This is a deceptive header, illegal under most commercial-email law, and it burns trust in a single send. Never worth it.
- “Free” stacked with money symbols and urgency. “FREE $$$ — ACT NOW” is a pile-up, not a word problem. Name the thing that is free: “Your first 3,000 emails, free”.
- “Act now”, “urgent”, “last chance” without a reason. Manufactured urgency is the most-imitated pattern in junk mail. State the actual deadline instead: “Pricing changes on 1 September”.
- “100% guaranteed”, “risk free”, “winner”, “congratulations”. Claims that no legitimate sender needs to make in a subject line. If the guarantee is real, it belongs in the body where you can qualify it.
- “Dear friend” and other non-greetings. Announces that you do not know who you are writing to. Either you know, or you skip the greeting.
- “Click here” in a subject line. Asks for the action before giving the reason. The subject sells the reason; the button asks for the click.
- Emoji as decoration. One can help a subject stand out. Three read as a sales flyer, render inconsistently across clients, and get announced individually by screen readers.
Length and rendering
Assume 35–45 characters of subject are visible on a phone and write the payload into that space. Anything after it is a bonus for desktop readers.
Do not open with your brand name — the sender field already says who you are, and the prefix pushes the meaning off the edge of the screen.
Every merge tag needs a fallback. A subject line that ships as “Hi , your report is ready” damages more trust than the personalisation was ever going to build.
“Keywords” in email are not SEO keywords
Nobody indexes your email. There is no keyword density to hit, no ranking to earn, and stuffing terms into the body does nothing except make it read worse. The only job the body's words have is to pay off the subject quickly.
The place email and search meet is on your website: the guide the email links to, the tool it recommends, the archive of the newsletter. That is where keyword research belongs — and where the same piece of writing can earn traffic for years instead of one afternoon.
Common questions
Does the word “free” send emails to spam?
No. Filters weigh authentication, reputation and recipient behaviour far above vocabulary. “Free” appears in enormous volumes of wanted mail. It only becomes a problem stacked with caps, exclamation marks and urgency on a domain with no reputation.
Do emojis in subject lines improve open rates?
Results are mixed and audience-specific. Test it on your own list, keep it to one, and never let an emoji carry meaning the words do not also carry.
Should I put the recipient's first name in the subject line?
Only if it is genuinely relevant — and only with a fallback value. The measurable lift is small, and a broken merge tag is worse than no personalisation at all.
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