The old advice was about keywords: avoid "free", avoid excessive punctuation, do not use the word "guarantee". That advice describes a filtering generation that has largely been replaced.
Modern inbox filtering evaluates sender reputation, engagement history, structural patterns across many recipients and, increasingly, whether the message reads as bulk regardless of its wording. You cannot word your way past a system that is looking at whether four hundred near-identical messages left one domain this morning.
What the filters are actually reading
Roughly in order of weight: how recipients have historically treated mail from this domain, whether this specific recipient has ever engaged, structural similarity across the batch, link and tracking-pixel patterns, and only then content.
Which means most "copywriting for deliverability" advice is optimising the smallest term in the equation.
Write for the reply, not the filter
The useful reframing: the strongest deliverability signal you can generate is a reply. Replies tell the provider this is wanted mail, and they compound — a domain with a healthy reply rate gets more latitude on everything else.
So the copy goal is not "avoid triggering the filter". It is "get a response from a smaller number of better-chosen people", which happens to be the same thing.
The strongest deliverability signal you can generate is a reply.
Six things that hold up
- Short. Under ninety words. Long cold mail is not read, and length correlates with bulk patterns.
- Plain text. No images, no HTML layout, no signature graphics. Cold mail should look like mail a person typed.
- One link at most, ideally zero in the first touch. Link count is a real signal and the first email rarely needs one.
- No open tracking on cold. Pixels are increasingly flagged, the data has been unreliable since mail privacy protection shipped, and you are trading a real deliverability cost for a metric you should not be trusting anyway.
- A specific, observable opening. Something true about them that took effort to find. This is the one part worth spending AI on.
- A small ask. "Worth a conversation?" outperforms a calendar link in first touch, consistently, because it is answerable in four words.
Vary the structure, not just the fields
Mail-merge variation fools nobody. Four hundred messages with the same three sentences and a different company name are structurally identical, and structural similarity is precisely what batch detection looks for.
Real variation means different sentence counts, different orderings, different openings — genuinely different messages that happen to make the same argument. This is where language models earn their place in the stack, and it is a much better use of them than writing the pitch.
The test that matters
Seed inboxes across the major providers, send the actual campaign to them alongside the real list, and check placement — inbox, promotions or spam — per provider. Do it every campaign, not once at setup. Placement drifts, and reply rate will tell you eventually but placement tells you today.