What are “stop words” and do they matter for filtering?

If you've ever deleted "free" from a subject line just in case it trips a spam filter, you've been following advice that hasn't been accurate for over a decade. Stop words, terms like "free," "help ensure," "winner," and "act now," date back to a simpler era of spam filtering, and their impact today is a fraction of what the folklore suggests.

Stop words were a real concern in the early 2000s when filters used rule-based keyword matching. Flag enough suspicious words in a message and it got blocked. That's not how modern filtering works. Gmail, Outlook, and the major inbox providers now use machine learning models that evaluate hundreds of signals at once: your sending history, your authentication setup, your engagement rates, and whether your emails resemble messages users previously marked as spam.

"Free" in a subject line from a sender with strong sender reputation and clean authentication is fine. The same word from a sender with poor engagement and a list full of inactive addresses is a problem, but not because of the word. It's a problem because of everything else. Filters that see "free" from a trusted sender treat it like a promotion. Filters that see it from a reputation-damaged sender treat it as confirmation of what they already suspected.

That doesn't mean anything goes. Certain phrases in certain contexts, especially those designed to mimic system alerts or create financial urgency, still correlate with spam classifications. But the right answer isn't sanitizing your vocabulary. It's building the signals that make your content trustworthy: valid SPF, DKIM, and DMARC records, a list built on confirmed opt-ins, and regular list hygiene to remove subscribers who've stopped engaging.

If you're worried about a specific campaign, test it. Send to a seed list across multiple providers and check placement before your main send. That'll tell you far more than counting adjectives in your subject line.