Subject line frameworks:

• Curiosity gap — "[Topic]: the part nobody talks about" (Engaged subscribers)
• Specific benefit — "How to [achieve X] in [timeframe]" (New subscribers)
• Number list — "[N] [things] that [outcome]" (Broad audience)
• Contrarian — "Why [common belief] is wrong" (Opinionated niche)
• Story hook — "The time I [unexpected event]" (Personal brand)
• Urgency — "[Breaking/New]: [what happened]" (Time-sensitive)
• Question — "Are you [doing common mistake]?" (Problem-aware audience)

Subject line rules:

• Keep it under 50 characters (mobile truncation)
• Avoid ALL CAPS and excessive punctuation (spam filters)
• Personalization tokens (e.g., subscriber's first name) often increase opens by a measurable margin
• A/B test subject lines: recommended split is 10% variant A, 10% variant B, wait 1–2 hours, then send the winner to the remaining 80%
• Never use "Newsletter #47" — that is a filing system, not a hook

Preview text (preheader)

The first ~90 characters of body text that appear below the subject line in most email clients. This is free real estate that most newsletters waste.

Good preheader patterns:

• Extend the curiosity gap: Subject "AI agents: the part nobody talks about" → Preview "Hint: it's not the prompts."
• Promise specifics: Subject "5 prompt patterns" → Preview "Including the one that saved us 40% on API costs."
• Contrast: Subject "Why fine-tuning is wrong" → Preview "RAG + evals beats it in 90% of use cases. Here's the data."

Newsletter structure

The reliable newsletter anatomy:

1. Hook (2–3 sentences) → Why should the reader care about this issue? What is at stake?

2. Main insight (300–800 words) → The core value: framework, story, analysis, or tutorial → Include proof: data, screenshots, code, personal experience

3. Takeaway (2–3 sentences) → The one thing the reader should remember or do

4. Bonus section (optional, 100–200 words) → Curated links, tools, or a secondary insight

5. CTA (1 sentence) → Reply, share, click, or act on the takeaway

6. Footer → Social links, archive link, referral prompt

Audience segmentation

Not all subscribers want the same thing. Segment by behavior, not demographics:

• Power readers — Open >80% of issues, click links, reply. Give them early access and exclusive content.
• Regular readers — Open 40–80%, occasional clicks. Optimize the main newsletter for them.
• Passive readers — Open 10–40%, rarely click. Use re-engagement campaigns and shorter issues.
• Ghost subscribers — Open <10% over 3 months. Run a win-back email, then clean the list if no response.

Growth tactics

Organic growth levers (practical):

• Social cross-promotion — Link to the newsletter in social bios; post hook paragraphs as teasers.
• Content teaser — Post the hook paragraph on social and link to the full issue to drive signups.
• Referral program — Simple referral asks or a small gated bonus for shares can scale signups.
• Guest swaps — Write for another newsletter; arrange swaps with aligned authors.
• Gated content — Use only when the asset provides clear, immediate utility (templates, spreadsheets).
• Conference mentions — Briefly call out your newsletter in talks or panels.
• SEO (Substack native) — Substack posts are indexed; use searchable titles and evergreen content.
• Recommendation network — Opt into Substack recommendations with aligned newsletters.

Indie Hackers observations: successful launches often combine multiple channels (HN, Product Hunt, LinkedIn, Reddit) with deliberate content that targets those audiences; cold outreach and interviews can seed an initial list (see Indie Hackers case summaries for tactics and channel experiments).

Retention strategies

Reduce churn by:

• Sending consistently (same day, same time)
• Honoring the value contract (never send filler)
• Cleaning the list quarterly (remove ghosts, improve deliverability)
• Asking for replies (replies signal "real person" to mail providers)
• Providing subscriber-exclusive value (not just repackaged social content)
• Running a short exit survey when people unsubscribe

Deliverability and inbox indicators (DMARC, BIMI, Apple Branded Mail)

Recent changes in inbox display and authentication matter for trust and open rates. Implementing the basics protects deliverability and unlocks brand indicators in some clients.

• DMARC: To improve deliverability and to enable BIMI/brand indicators, set a DMARC record on your sending domain. For BIMI visibility some providers require a policy of p=reject or p=quarantine — adjust and test carefully (Resend's BIMI docs outline this requirement).

• BIMI/CMC/VMC: The BIMI standard now accepts Common Mark Certificates (CMC) in some providers (notably Gmail). A CMC lets you show a logo without owning a trademark if you can prove at least 12 months of public logo use. Alternatively, Verified Mark Certificates (VMC) require trademark verification but may show a verified checkmark in certain clients. See Resend's BIMI guide and blog for implementation steps and certificate options.

• Apple Branded Mail: Apple introduced Branded Mail (via Apple Business Connect) which can display a brand image for verified organizations on Apple Mail across modern Apple devices; the verification path is through Apple Business Connect rather than a VMC.

Practical checklist:

• Add SPF, DKIM, and a DMARC record to your sending domain (start with p=none for monitoring, then move to quarantine/reject when comfortable).
• If you want a logo in inboxes: choose CMC (if eligible) or VMC and follow the BIMI guidance; Apple Branded Mail uses Apple Business Connect.
• Use an ESP/transactional provider with clear docs (Resend provides step-by-step BIMI/DMARC documentation).

Sources: Resend BIMI documentation and Resend blog explaining CMC and Apple Branded Mail (implementation prerequisites: DMARC policy and verifiable logo usage or trademark).

Resend integration

Use Resend for welcome sequences, re-engagement emails, transactional notifications, and custom designs that you don't want to send through Substack's native system. If you implement BIMI or branded avatars, Resend's docs include implementation steps and requirements (DMARC, CMC/VMC, Apple Business Connect).

Example (Node.js):

import { Resend } from 'resend'; const resend = new Resend(process.env.RESEND_API_KEY); await resend.emails.send({ from: 'newsletter@yourdomain.com', to: subscriber.email, subject: 'Your weekly digest', react: <WeeklyDigestEmail data={digestData} />, });

Guidance: Use Resend for transactional and event-driven messages only if you can guarantee subscribers won't receive duplicate content from Substack.

Workflow

Step 1 — Define the value contract

Write one sentence: "Subscribers get [specific value] delivered [cadence]."

Example: "Subscribers get one actionable AI engineering insight with code examples, delivered every Wednesday."

Step 2 — Build the subject line library

Create 20+ subject line templates using the frameworks above. Score each by: curiosity, specificity, and audience fit.

Step 3 — Draft the issue

Follow the newsletter anatomy: Hook → Main insight → Takeaway → Bonus → CTA → Footer.

Step 4 — Write the subject line and preheader

Draft 3 subject line options. Pick the one with the strongest curiosity-to-specificity ratio. Write a preheader that extends, not repeats, the subject line.

Step 5 — A/B test

Send variant A to 10% of the list, variant B to 10% of the list, wait 1–2 hours, then send the winner to the remaining 80%.

Step 6 — Analyze and iterate

After each issue, record: open rate, click rate, reply count, unsubscribe count, and top-clicked link.

Examples

Example subject line A/B test

Issue topic: How to set up AI agent evals

Variant A: "How to set up evals in 30 minutes" Variant B: "Most AI teams skip evals. Here's why that's expensive." Test: 10% A, 10% B, winner to 80%.

Example re-engagement email

Subject: "Still interested? (Honest question)" Preview: "I noticed you haven't opened in a while. No hard feelings either way."

Body: Hey [First Name],

I noticed you haven't opened the newsletter in a few months.

No guilt trip — inboxes are overwhelming, and I only want to be in yours if I'm earning the space.

Quick question: what would make this newsletter worth opening again?

→ More code examples → Shorter issues → Different topics (reply and tell me) → Honestly, I'd rather unsubscribe (no hard feelings — link below)

Either way, thanks for being here.

[Unsubscribe link]

Example weekly metrics log

Issue #23: "Why I Deleted My RAG Pipeline" Sent: 3,200 subscribers Open rate: 47% (target: 40%+) Click rate: 12% (target: 8%+) Replies: 14 Unsubscribes: 3 (0.09%) Top clicked link: GitHub repo link (38% of clicks)

Analysis: - Contrarian subject line drove above-average opens - Code repo link was the primary CTA driver - Reply count suggests strong engagement with the topic - Unsubscribe rate well below 0.5% threshold

Action: - Create a follow-up issue expanding on the migration process - Test a "curiosity gap" subject line next week for comparison

Decision tree

1. Do I have a clear value contract? No → Step 1. Yes → continue.
2. Am I sending consistently? No → set a cadence and stick to it for 4 weeks before optimizing anything else.
3. Is my open rate below 30%? Yes → focus on subject lines and preheaders; track click rate as a more reliable engagement signal where privacy protection inflates opens. No → continue.
4. Is my click rate below 5%? Yes → improve CTAs and the main insight section. No → continue.
5. Am I growing subscribers? No → implement 2–3 growth levers from the growth tactics list. Yes → continue.
6. Is churn above 1% per issue? Yes → run re-engagement campaign and exit survey. No → continue.
7. Am I segmenting my audience? No → start with the 4-segment model. Yes → optimize content per segment.

Edge cases and gotchas

• Open rate inflation — privacy protections (e.g., Apple Mail Privacy Protection) can pre-load tracking pixels and make open rates unreliable. Track click rate and replies as stronger signals of engagement.
• Substack discovery tax — Substack's recommendation network drives growth but also attracts low-intent subscribers. Segment and clean aggressively.
• List hygiene — email providers penalize senders with high bounce rates. Remove invalid emails monthly and ghost subscribers quarterly.
• Reply-to engagement — asking for replies boosts deliverability (signals "real person" to spam filters), but you must actually reply back.
• Unsubscribe friction — never make unsubscribing hard. A clean list with engaged subscribers outperforms a bloated list every time.
• Weekday vs. weekend — B2B newsletters perform best Tue–Thu morning. B2C can work weekends. Test your specific audience.
• Subject line fatigue — rotate through 3–4 frameworks to avoid predictable patterns.
• Preview text gotcha — if you do not set a preheader explicitly, email clients may pull the first body text, which might be "View in browser" or navigation links.
• Resend + Substack overlap — if using both, ensure subscribers do not receive duplicate content. Use Resend for transactional/event-triggered messages only when you can guarantee no duplication.
• Paywall timing — Substack paid subscriptions work best after 20+ free issues with consistent quality. Do not gate content before you have proven the value.

Evaluation criteria

• Open rate: Poor <25% | Acceptable 35–45% | Strong >50%
• Click rate: Poor <3% | Acceptable 5–10% | Strong >12%
• Reply rate: Poor 0 replies | Acceptable 3–10 | Strong >15
• Unsubscribe rate per issue: Poor >1% | Acceptable 0.3–0.5% | Strong <0.2%
• Subscriber growth (monthly): Poor flat | Acceptable 5–10% | Strong >15%
• Consistency: Irregular | Weekly | Weekly + bonus issues
• Subject line A/B testing: Never | Monthly | Every issue
• List hygiene: Never cleaned | Annually | Quarterly
• Segmentation: None | 2 segments | 4+ behavioral segments