What a cold email subject line has to do
A subject line has one job: get a stranger to open a message from someone they do not know. It does that by being specific, honest and short. It does not need to be clever. The best performing cold subject lines usually look like something a colleague would type in a hurry: a company name, a problem, a question with a real answer.
Everything this tester measures comes back to that. Length matters because a phone shows about 41 characters. Capitalisation matters because Title Case and ALL CAPS look like marketing. Spam words matter because they trigger filters and, more often, the reader's delete reflex.
Length and the mobile cut
Gmail on a phone shows roughly 41 characters of a subject before truncating, and the desktop web client shows around 70 before the preview text takes over. Because most first opens happen on a phone, the words after position 41 are effectively invisible. Put the hook, usually the company or the problem, at the start.
Too long:
Following up on my previous email regarding your outbound sales infrastructure
Fits mobile:
{{company}} outbound setup
the SDR onboarding gap
question about your 40 domainsBetween 30 and 50 characters and 3 to 7 words is the range that displays fully and still says something. Shorter can work when the words are concrete; a single vague word such as introduction almost never does.
Patterns that hurt and patterns that help
- Fake Re: or Fwd: prefixes fail outright. They trick the reader, breach anti-spam law in most countries, and a tricked reader does not reply.
- Exclamation marks, emoji and repeated punctuation mark the email as promotional.
- Lowercase or sentence case reads like a real note. Title Case reads like a press release.
- A number, a merge tag or a named trigger event (a hire, a launch, a funding round) makes the line specific to the reader.
- Overused openers such as quick question or checking in are not spam, but they are invisible because everyone uses them.
Once the subject passes, remember it is only the first filter. The sending domain still has to authenticate and the body still has to earn a reply. InboxOne handles the domain side automatically, which leaves the subject and the copy as the only things you need to test.
