Postmark has delivered billions of transactional emails over 15+ years. This skill distills that expertise into actionable guidelines for building reliable, compliant, high-deliverability email systems.
Quick Reference
| Topic | Use When |
|---|---|
| Deliverability | Setting up SPF/DKIM/DMARC, warming a new domain, diagnosing delivery issues |
| Compliance | Building unsubscribe flows, handling GDPR/CAN-SPAM/CASL requirements |
| Transactional Design | Designing welcome emails, password resets, receipts, alerts |
| List Management | Handling bounces, suppressions, list hygiene |
| Testing | Testing safely without hurting sender reputation |
| Sending Reliability | Idempotency, retry logic, rate limits |
Deliverability Fundamentals
The three authentication records every sending domain must have:
| Record | Purpose | Priority |
|---|---|---|
| SPF | Authorizes servers to send as your domain | Required |
| DKIM | Cryptographically signs emails to prove authenticity | Required |
| DMARC | Policy for handling SPF/DKIM failures | Required |
With Postmark, DKIM is configured automatically when you verify a sender domain. SPF and DMARC must be set up in your DNS.
See references/deliverability.md for DNS setup, reputation factors, and domain warm-up guidance.
Transactional vs. Broadcast Email
Never mix transactional and broadcast email in the same sending stream. They have different delivery characteristics, compliance requirements, and reputation profiles.
| Type | Examples | Compliance | Unsubscribe Required |
|---|---|---|---|
| Transactional | Password resets, receipts, alerts, notifications | CAN-SPAM exemption possible | No (but good practice) |
| Broadcast | Newsletters, promotions, announcements | CAN-SPAM, GDPR, CASL apply | Yes — legally required |
Postmark enforces this separation with Message Streams — use outbound for transactional, broadcast for marketing.
See references/compliance.md for CAN-SPAM, GDPR, and CASL requirements.
Transactional Email Design
Good transactional emails are:
- Expected — The recipient triggered this email
- Timely — Sent immediately after the triggering event
- Actionable — One clear call to action
- Plain — Minimal design; content over decoration
Common transactional email types and their essential elements:
| Email Type | Must Include | Avoid |
|---|---|---|
| Welcome | Product name, next step CTA, support contact | Marketing upsell on day 1 |
| Password reset | Expiry time, ignore-if-not-you notice, support link | Long copy |
| Receipt / Invoice | Line items, total, billing address, support | Promotional content |
| Shipping notification | Tracking link, estimated delivery, items | Unrelated promotions |
| Security alert | What happened, when, action required, how to secure | Panic-inducing language |
See references/transactional-design.md for design patterns, copy guidelines, and HTML email best practices.
List Health
Sending to invalid, inactive, or unengaged addresses is the leading cause of deliverability problems.
Key rules:
- Remove hard bounces immediately and permanently
- Suppress spam complaints immediately — never re-add
- Re-permission lists older than 12–18 months before mailing
- Never purchase or rent email lists
- Validate addresses at the point of collection
See references/list-management.md for suppression strategies, list hygiene schedules, and re-engagement workflows.
Testing Safely
Never test with real addresses at consumer providers (gmail.com, yahoo.com, etc.) — it damages sender reputation.
| Method | How | Use For |
|---|---|---|
| API test token | Use POSTMARK_API_TEST as your server token |
Validating API calls in CI/development |
| Black hole | Send to test@blackhole.postmarkapp.com |
Functional testing — appears in activity |
| Sandbox server | Create a dedicated sandbox server in dashboard | Full send pipeline without delivery |
| Bounce testing | hardbounce@bounce-testing.postmarkapp.com |
Testing bounce webhook handlers |
See references/testing.md for full testing setup and domain warm-up schedules.
Sending Reliability
Production email systems need idempotency keys, retry logic, and rate limit handling to avoid duplicate sends and silent failures.
See references/sending-reliability.md for idempotency patterns, retry strategies, and rate limit handling.
Notes
- Postmark is purpose-built for transactional email — use it for triggered 1:1 emails, not bulk marketing
- Deliverability is not just about authentication — it's about sending wanted email to engaged recipients
- A single spam complaint from a real user is more damaging than 1,000 hard bounces
- Monitor your bounce rate (keep below 2%) and spam complaint rate (keep below 0.04%)