The three things a reader sees first
Before anyone opens an email they see the sender name, the subject line and the preview text. Those three decide the open. A recognisable, consistent sender name matters more than most people think — people open emails from names they trust.
Treat the subject and preview as a pair: the preview should extend the subject, not repeat it.
Subject-line principles that work
Clarity beats cleverness. A subject that tells the reader what's inside and why they should care will usually beat a vague or gimmicky one. Keep it short enough to survive mobile truncation, make it specific, and, where it's true, make it feel timely or relevant to them right now.
Curiosity works, but only if the email pays it off — a subject that over-promises trains people to stop opening.
Avoid the spam triggers
ALL CAPS, rows of exclamation marks, and classic spam phrases ('FREE!!!', 'act now', 'risk-free') can hurt both deliverability and trust. So can a subject that reads as clickbait. Write like a person emailing another person, not a billboard.
Open rate isn't the whole story
A high open rate on a subject that misleads is a hollow win — the clicks and conversions won't follow, and your list learns to ignore you. And list health matters more than any single subject line: a clean, engaged list of people who want your email will out-open a bigger, colder one every time.
Note too that privacy features now inflate reported opens, so treat open rate as a directional signal and watch clicks and replies for the real story.