What makes a good email subject line?
The short answer
Short (around 40 characters), concrete, and curiosity-sparking without overpromising. Promise what the email actually delivers, because clickbait costs you opens over time. Test variants with your tool's A/B feature. Data beats every rule of thumb.
In detail
The subject line decides, together with your sender name, whether an email gets opened. The sender matters more than most people think. A consistent, recognizable name (yours or your brand) does more than any clever line. The subject line itself only has to generate enough curiosity for a click, among the right people.
What works is specificity ("3 pricing mistakes in your proposals" beats "Our latest tips"), a hint of tension the email actually resolves, and numbers or a personal tone where it fits. What backfires is ALL CAPS, rows of emoji, and promises the content does not keep. Recipients learn fast and your next emails stay closed.
The honest answer is to test. Almost every tool (Mailchimp, MailerLite, GetResponse, ActiveCampaign) supports A/B testing. Two subject lines go out, the winner goes to the rest of your list. Ten sends in, you will know more about your audience than any article can tell you. And do not forget the preview text, the gray snippet after the subject. That is free extra persuasion space.