Subject length and spam flags (2026)
Industry guidance still clusters around short, specific subjects: roughly 6–10 words and often ~30–50 characters so mobile clients do not clip the hook (Braze, Benchmark, Mailsoftly).
There is no single magic “banned word list,” but ESP spam checkers still flag patterns: ALL CAPS chunks, multiple exclamation marks, and stacked phrases like free, act now, guarantee, click here (GetResponse / Benchmark / Drip summaries).
SubjectLab scores length and common spam-ish phrases as lab heuristics — not open-rate or inbox guarantees. Always A/B in your ESP.
Refs: Braze · Benchmark · Mailsoftly 2026.