Warming up
If the subject line doesn't earn the open, nothing else in your email matters. Here's what works, what to avoid, and why short and specific wins.
The subject line is the gate to the whole email. It has one job: earn the open without overpromising. The best cold email subject lines look like something a colleague would send — short, lowercase, specific — not like marketing.
Spammy subject lines don't just lower opens — they can route the whole email to spam. But word choice is a minor factor next to sender reputation: a warmed mailbox with a plain subject lands; a cold mailbox spam-folders regardless of how clever the subject is.
Short, specific, and plain — something that reads like a real person wrote it to one person. Reference their company or role, create mild curiosity, and never overpromise something the email doesn't deliver.
They can. Spammy words, excessive punctuation, and all caps can trigger filters. But subject wording is a small factor compared with sender reputation and mailbox warmup, which are what mainly decide inbox vs spam.
Gmail's mobile app truncates subject lines around 33-40 characters depending on screen width and sender name length; Outlook mobile cuts off closer to 30. Keep the load-bearing words in the first 30 characters.
Test in pairs with only one variable changed — same body copy, same send time, one subject line difference — and send at least 100-150 emails per variant before drawing conclusions. Judge the winner on reply rate, not open rate.
Run a free deliverability check on your sending domain — SPF, DKIM, DMARC, and spam-trap risk — before you send another outreach sequence.
Warmerly builds the sender reputation that gets your emails seen in the first place. Start free.