Subject line
The visible text shown in the inbox before opening — the primary lever on open rate.
The subject line is the visible text shown in the recipient's inbox alongside the From: name and preheader, before the email is opened. It's the primary lever on open rate — historically the most-tested element in email marketing.
Effective subject lines vary by context. Cold outreach skews toward specificity ("Quick question about {{company}}'s onboarding"). Marketing campaigns skew toward benefit-driven framing or curiosity gaps. Transactional emails skew toward clarity ("Your order is shipped"). The same subject-line pattern that wins in one context will lose in another.
Common subject-line mistakes: trying to be clever instead of clear, faking urgency ("LAST CHANCE" when there's a week left), using ALL CAPS or excessive exclamation points (spam triggers), and writing the subject line without a coordinated preheader (wastes the second-line real estate). Mobile clients show 30–40 characters; desktop shows 60+. The first 30 characters carry most of the load on mobile-heavy audiences.