Subject line length and spam flags: a practical 2026 checklist
Inbox truncation and filter words still kill opens. SubjectLab scores length and common spam phrases before you A/B.
Mobile inboxes often clip past ~40–50 characters. Filter-sensitive words like “free” or “act now” raise risk without guaranteeing spam-folder placement.
SubjectLab is decision-support — not a deliverability lab. Pair it with your ESP’s own tests. See also industry guides from major ESPs on subject hygiene.