How-To

How to write cold email subject lines that don't trigger spam filters

subjectline_sarah · 2026-04-04T00:00:00Z

I have had subject lines tank entire campaigns because a single word triggered a spam filter. After testing 200 plus subject lines across 100K sends, here

subjectline_sarah · 2026-04-04 · 1,870 views

I have tanked entire campaigns with bad subject lines. Not bad in a "low reply rate" sense. Bad in a "every email went to spam" sense. After testing over 200 subject lines across 100,000+ sends, here are the rules I follow without exception.

Rule 1: Keep it under 50 characters. Short subject lines look personal. Long ones look like marketing. When you see a subject line that says "Quick question about your Q2 pipeline strategy and how we can help optimize it" you know immediately it is automated. Compare that to "quick question" or "[Company] + outbound." Short, specific, and it could have been written by a real person.

Rule 2: No ALL CAPS. Not even one word. "FREE" triggers filters. "URGENT" triggers filters. "IMPORTANT" triggers filters. Even capitalizing a word for emphasis like "QUICK question" increases your spam score. Just do not do it. Lowercase or normal capitalization only.

Rule 3: No exclamation marks. "Great opportunity for you!" lands in spam more often than "Great opportunity for you." The exclamation mark is one of the oldest spam signals in email filtering. Spam filters were trained on decades of emails that said things like "Act now!" and "Limited time offer!" Every exclamation mark in your subject line nudges you closer to spam.

Rule 4: Avoid trigger words. These words have been flagged by spam filters for years and they still cause problems:

Free, guarantee, limited time, act now, exclusive, winner
$$, earn money, no cost, save big, discount
Urgent, important, action required, immediate
Click here, apply now, order now, buy now

Some of these seem obvious. But I have seen experienced cold emailers use "free" in subject lines and wonder why inbox placement dropped. The word itself is a spam signal regardless of context.

Rule 5: Match the email tone. If your email body is casual and conversational, your subject line should be too. A formal subject line on a casual email looks mismatched, and some spam filters flag that inconsistency. Keep it congruent.

Subject lines that work consistently for me:

"quick question about [specific thing]"
"[Company] + [your company/topic]"
"[mutual connection] suggested I reach out"
"[First name], saw your [specific thing]"
"idea for [company]"

These all share the same qualities: short, lowercase or sentence case, no trigger words, and they sound like a real person wrote them.

How to test before sending: Run your subject line through the PuzzleInbox spam word checker before launching any campaign. It flags trigger words and gives you a spam score. Takes 10 seconds and catches problems before they cost you a campaign. You can also send a test email to a seed list on GlockApps and check whether the subject line alone is causing spam placement.

Subject lines feel like a small thing. They are not. I have seen campaigns go from 1.2% reply rate to 3.5% reply rate just by swapping a spammy subject line for a clean one. The emails were identical. The subject line was the only variable. Get this right and you give the rest of your email a chance to actually be seen.