A subject line does one job: it earns the next two seconds. Everything else happens in the body, so anything that distracts from being opened is a cost.
What works
- Short — three to six words, because it may be truncated on mobile
- Specific — mention the company, the role or the market
- Lowercase and plain, the way a colleague would type it
- No hype — no exclamation marks, no emoji, no capitalised words
- No sales vocabulary — nothing that sounds like marketing
Plain subject lines outperform clever ones because they look like a normal work email. That is the entire objective.
Formulas worth testing
- The question — a quick question about your pipeline
- The referenced signal — saw the new warehouse in Dallas
- The direct value — one thousand verified emails for your target market
- The soft close — closing the file on this
- The peer reference — how agencies like yours build lists
Pick one formula per test, not a mixture. A test with two variables tells you nothing.
Words that quietly reduce opens
Free, guarantee, limited time, act now, exclusive offer and anything in all capitals all read as bulk mail. So do long dash-separated subject lines and strings of numbers. If a subject line would look at home in a banner advert, it will underperform in an inbox.
Test one variable at a time
Subject line A against B, same list, same offer, same day, same sender. Anything else and the result is noise.
Send enough volume to matter — at least three hundred per variant — and measure reply rate rather than open rate. Open rates are increasingly unreliable due to privacy features that pre-fetch images, while replies are unambiguous.
What to do with a winner
When a subject line wins, do not retire it. Rotate it across new segments and revisit it every quarter, because inbox behaviour shifts. Keep a documented library of your best-performing lines so the knowledge stays with the business rather than with one person.