Postal

Fair

Agent Native Score

Free TierAPI Key AuthOpenAPI Spec

Postal is an open-source mail server and email delivery platform that allows developers to send and receive emails programmatically with full control over infrastructure and compliance.

Categories: Email · Infrastructure · Communications

#18 of 30 in Email · #26 of 49 in Infrastructure · #32 of 63 in Communications

Checklist Breakdown

13 of 33 checks passed. 14 unscored.

Discovery 63%

Can an agent find and understand this tool without a web search?

✓ Published OpenAPI/Swagger spec

✗ Has llms.txt or llms-full.txt

✗ Has an MCP server (official or well-maintained)

✗ MCP server listed in a public registry

✓ API reference docs are publicly accessible

✓ Docs include runnable code examples

✓ Has a public changelog or release notes

✓ Has a public status page

Auth & Onboarding 50%

Can an agent create an account and get credentials without human intervention?

✗ Signup does not require CAPTCHA

✗ Signup does not require phone verification

✓ Supports API key auth (not only OAuth)

✗ API key obtainable without manual approval

✓ No mandatory billing info to start

✓ Can sign up without creating an organization

Pricing 100%

Can an agent operate autonomously without upfront payment or contracts?

✓ Has a free tier

✓ Usage-based pricing available

✓ No minimum contract or commitment

✓ Pricing page is public (no 'contact sales')

✓ Free tier sufficient for testing (not just a trial)

Agent Tooling Not yet scored

How well does the API work for non-human consumers?

— SDK available in 2+ languages

— Structured error responses (JSON with error codes)

— Idempotency support on write endpoints

— Pagination on list endpoints

— Webhook/event support

— Sandbox or test mode available

— Rate limit headers in responses

— Consistent REST resource naming

Reliability Not yet scored

Does the tool fail gracefully when an agent makes a mistake?

— Meaningful error messages (not just 500)

— 429 responses include Retry-After header

— Documented uptime SLA (99.9%+)

— Graceful degradation under rate limits

— Request IDs in responses for debugging

— API versioning supported

Reviewer Notes

Postal has decent API tooling with REST endpoints and reasonable documentation, plus a free tier for experimentation. However, it lacks an MCP server and llms.txt for discoverability, making initial discovery harder for agents. Account creation requires manual signup through a web interface with potential email verification steps—agents cannot programmatically bootstrap instances. The open-source self-hosted model is excellent for long-term reliability and control but adds deployment complexity that agents must handle. API responses are structured and usable, though error handling documentation could be more comprehensive for agent error recovery paths.

Postal

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