A subject line has one job: give the right reader a clear reason to open. Everything else people obsess over is downstream of that, and most of the advice floating around is about tricks rather than reasons.
Say what is inside
Before the clever wording, answer what the reader gets. A specific problem, outcome or question beats hype every time. "Newsletter" tells someone nothing. "How to recover more abandoned carts this week" tells them the topic and the payoff.
Obvious advice that somehow people ignore constantly.
Length is a mobile problem, not a rule
Aim for 30 to 50 characters. Not because a rule says so, but because Gmail on a phone cuts you off around 35 to 40 and most of your list is on a phone. Anything past that is a bonus, so put the part that earns the open at the front.
Stop worrying about spam words
The word "free" is not going to send you to spam. Filters have used machine learning and engagement signals for years, not a banned word list from 2006. We wrote about which spam trigger words actually matter, and the short answer is almost none of them.
What genuinely hurts is overpromising. Write a subject line the email cannot cash and you get complaints, and complaints are a real signal. That is the mechanism, not the vocabulary.
Personalization is not their first name
Everybody does the name thing, so the lift is basically gone. Behaviour beats identity: reference the product they looked at, the thing they bought, the plan they are on. That takes segmentation, which is why most people default to the name instead.
Keep the formatting boring
All-caps, three exclamation marks and a row of emojis read as desperate. One emoji at the front of a promotional send can help it stand out. Three looks like a phishing attempt. Test it on your own audience, because B2B lists tend to hate them and consumer lists often do not.
The preheader is half the pitch
The subject line does not work alone. It sits next to your preheader text, and most people leave that as "View this email in your browser", which is free space thrown away. Read the two together as one unit, the way someone scanning an inbox does. More on that in preheader text.
Test the ones that matter
A/B test launches, promotions and anything going to a big segment. Do not test the routine newsletter going to 800 people, because you will not reach significance and you will end up reading noise and calling it a finding. Subject lines are the biggest single lever in a test, so test them first and one at a time.
For the wider rules, we have a full guide on email subject line best practices, and one on increasing open rates for when the subject line is not the actual problem.