MCP

Subject lines

TL;DR

  • For sales outreach, default to short, lowercase, internal-style subjects — one to four words that read like a colleague's note.
  • Longer, descriptive subjects have data behind them only in PR and link-building outreach; the corpora differ, so don't transplant the advice.
  • Never fake "Re:" or "Fwd:" — deceptive subject lines are illegal in the US, and mail providers recognize the trick.

The subject line buys the open, and the two best-known datasets appear to contradict each other — until you notice they measured different kinds of email.

The schools

Short and internal-style (Gong data popularized through 30MPC, 2023-24 [1]; Jason Bay's practice [2]) — In the Gong reply data reported by 30MPC (2023-24), one-to-four-word subjects performed best, and reply odds fell off a cliff past roughly 13 words — about a 4.2x gap between the short band and the long tail. Bay pairs the length rule with a style rule: lowercase, curiosity-neutral, colleague-like subjects, which he reports opening at 40-50%+ (2023) — an open-rate claim, so weigh it with Measurement and iteration's tracking caveats. Corey Haines' open-source cold-email skill (2025-26) [3] lands in the same place: two to four words, lowercase, written the way a colleague would.

Fits when: selling to busy operators; mobile-heavy audiences; any standard B2B sales touch. · Avoid when: the recipient expects a formal or editorial pitch.

Recognizable by: subjects like "quick question", "pricing pages", "thursday?" — lowercase, no punctuation flourish, nothing that smells of marketing.

Longer descriptive (Backlinko, 12M-email corpus, 2019 [4]) — Backlinko's 2019 study of 12 million outreach emails found subjects of 36-50 characters earned roughly 24.6% more responses than the shortest ones. But that corpus is link-building and PR outreach — pitches to editors and site owners who triage by topic, not by sender. The real lesson is that the corpora differ: what wins in a journalist's inbox is not what wins in an operator's, so match the study population to your campaign before borrowing its numbers.

Fits when: PR, link-building, and partnership outreach — recipients scanning for topic relevance. · Avoid when: standard sales outreach; there a descriptive subject reads as a pitch and dies unopened.

Recognizable by: sentence-case subjects that state the topic plainly, such as "Data for your remote-work roundup".

Default if the user has no preference: short and internal-style — for sales outreach it has the stronger, population-matched data; go longer and descriptive only when the campaign is genuinely PR- or link-building-shaped.

The honesty line (no debate here)

A fake "Re:" or "Fwd:" prefix fabricates a relationship that does not exist. US law makes deceptive subject lines illegal, mail providers detect the pattern, and industry guides list it among the classic subject-line mistakes [5, 6]. A subject may be curious, plain, or blunt — it must be true. The wider compliance picture lives in Deliverability and compliance essentials.

References

  1. Gong (data) via 30MPC — 4 data-backed subject lines to get your cold emails opened (newsletter, ~2023-24)
  2. Jason Bay — internal-style subject formulas (podcast, Outbound Squad ep. 317, ~2023)
  3. Corey Haines — marketingskills cold-email skill (open-source skill pack, MIT, 2025-26)
  4. Backlinko — Email outreach study (12M-email study with Pitchbox, 2019)
  5. Instantly — Common cold email subject line mistakes (blog, n.d.)
  6. MarTech — Beyond clickbait: avoid deception in email subject lines (article, n.d.)