day3

Day3 provides email marketing and transactional email services billed by the number of emails sent. It offers unlimited subscribers, tiered pricing starting at $1 per month, and features such as deliverability tools, GDPR compliance, and one‑click unsubscribe. The platform targets startups and indie developers looking for a simple, usage‑based pricing model.

day3 is listed with 2 APIs on the APIs.io network. Tagged areas include Email Marketing, Transactional Email, Software-as-a-Service, and Startup Tools.

The day3 catalog on APIs.io includes 1 event-driven AsyncAPI specification, 1 JSON-LD context, and 1 Spectral governance ruleset.

day3’s developer surface includes authentication, signup flow, changelog, engineering blog, getting-started guide, documentation, pricing, and 24 more developer resources.

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51.8/100 developing Agent 44/100 agent ready Full breakdown ↓
scored 2026-11-06 · rubric v0.27.0 · Show this score on your site →

AccessPaid

2 APIs

Email Marketing Transactional Email Software-as-a-Service Startup Tools

Kin Score

Kin Score How this is scored →
scored 2026-11-06 · rubric v0.27.0

Composite quality — 51.8/100 · developing

Contract Quality 7.2 / 21

Developer Ergonomics 8.5 / 17

Access Clarity 15.7 / 17

Operational Transparency 3.8 / 11

Contract Governance 2.2 / 10

Discoverability 4.1 / 9

Regulatory Posture 5.3 / 15

Agent readiness — 44/100 · agent ready

Machine-Readable Contract 18 / 18

Agentic Access Contract 0 / 10

Documented Reversibility 0 / 6

MCP Server 0 / 12

Machine-Readable Auth 10 / 10

Idempotency 9 / 9

Stable Error Semantics 0 / 8

Request/Response Examples 7 / 7

Rate-Limit Signaling 0 / 7

Typed Event Surface 6 / 6

Agent Skills 0 / 5

Well-Known Catalog 0 / 4

Consent & Bot Identity 0 / 3

A2A Agent Card 0 / 8

Dry-Run / Simulate Mode 0 / 4

Delegated User Identity 6 / 6

Protected Resource Metadata 5 / 5

Registration Without a Human 6 / 6

Agentic Commerce Well-Known Document 0 / 5

Pagination 5 / 5

Distinct Operations 0 / 3

MCP Tool Quality 6 / 6

Regulatory Posture applies to this provider. Its tags matched the Horizontal (data, software, accessibility, platform) regime, so Regulatory Posture carries 15 points of the composite. If this regime is wrong for your business, say so on your provider repo — the applicability map is public and we will correct it.

Create-or-Update Ergonomics could not be measured. We hold no machine-readable contract for this provider to read, so there is nothing to measure a write surface against. Excluded rather than scored zero: never-measured and measured-empty are different facts. Publishing an OpenAPI is what makes this facet — and several others — scorable at all.

The six quality facets above are damped to 85 points between them, because the conditional facet above carries the other 15. That is why each facet's contribution is shown against a damped maximum: raising a quality facet moves the composite by 85% of its nominal weight, not 100%. The full arithmetic is at apis.io/rating/.

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Agent readiness — checks not met, largest first (exact, from this profile):

MCP Server +12 pts — deployment.mode + probe verdict on the mcp/ manifest, not the pointer; an unprobed slug falls back to authorship
Agentic Access Contract +10 pts — agentic_access artifact present (x-agentic-access), graded by author
Stable Error Semantics +8 pts — OpenAPI $refs a shared error schema across 4xx/5xx responses (verified), else common[].type includes ErrorCatalog / ErrorCodes / DeclineCodes
A2A Agent Card +8 pts — a2a/-a2a.yml manifest, graded on conformance.grade against A2A 1.0.0
Rate-Limit Signaling +7 pts — OpenAPI responses document X-RateLimit-* / RateLimit headers (verified), else a rate_limits artifact or RateLimits link (documented)
Documented Reversibility +6 pts — a reversal operation in the contract, graded up when a window is documented too

6 more unmet checks in the Agent Readiness table above.

Kin Score — facets with the most composite points still available (weight × headroom):

Contract Quality scores 33.8 — up to +16.6 composite points
Developer Ergonomics scores 50.0 — up to +10.0 composite points
Contract Governance scores 22.0 — up to +9.4 composite points

Each check links to what satisfies it. Publish the artifact, or tell us where it already lives, and the score moves on the next scoring run.

Improve this rating by publishing the missing artifacts — every area above can be raised, and the full rubric is at apis.io/rating/. Every facet and dimension name above is a link: it opens that measurement's own page — what it means, the exact checks that feed it, how the whole catalog distributes on it, and the providers at the top of it. This rating is computed from the provider profile at artifacts.apis.io/day3-app/apis.yml: open an issue on github.com/api-evangelist/day3-app to ask a question or point us at an artifact you publish. Submit an artifact on GitHub — free → Manage your own listing — the Influence plan, $499/mo →

Standards implemented 2

Standards this provider implements or carries. De facto ones are profiled and measured by the API Commons; formal ones come from the standards directory and are linked by the provider's own tags. The evidence column says how each claim was established — not that it was made.

OAuth 2.0 Protected Resource Metadata

measured — observed by APIs.io (verified)

Dynamic Client Registration

measured — observed by APIs.io (published)

APIs 2

Individual APIs this provider publishes, each with its own machine-readable definition.

Day3 API

Public API for Day3 email marketing and transactional email services. Documentation is available at the developer portal.

day3 API

day3 API as documented publicly: 19 operations. Contract generated from the documentation by API Evangelist (2026-11-05); not the provider's own document.

day3 API pricing

Prices published by day3, read 2026-11-05 from day3.app/pricing.

Plan	Price
1k	$1/mo
5k	$3/mo
10k	$5/mo
25k	$8/mo
50k	$14/mo
100k	$25/mo
250k	$60/mo
500k	$115/mo

All 9 plans, limits and sources →

Pricing Plans 1

Published pricing tiers and plan structures.

Day3 App Plans Pricing

9 plans

PLANS

Event Specifications 1

AsyncAPI definitions for this provider's event-driven and streaming APIs.

Day3 App Webhooks

ASYNCAPI

Semantic Vocabularies 1

JSON-LD contexts and semantic vocabularies used across these APIs.

Day3 App Context

19 classes · 34 properties

JSON-LD

Spectral Rules 1

Spectral governance rulesets for linting and validating these APIs.

day3 API Rules

43 rules · 41 errors 1 warnings 1 info

SPECTRAL

JSON Schema 6

Standalone JSON Schema definitions for this provider's data models.

GetEmailsIdResponse

10 properties

JSON SCHEMA

GetV1WebhooksResponse

11 properties

JSON SCHEMA

GetV1WebhooksWebhookidDeliveriesResponse

13 properties

JSON SCHEMA

GetV1WebhooksWebhookidResponse

11 properties

JSON SCHEMA

PostApiV1EmailsResponse

10 properties

JSON SCHEMA

PostYourEndpointRequest

4 properties

JSON SCHEMA

Security Posture 4

Authentication, domain security, vulnerability disclosure, and trust-center signals.

Day3 App Authentication

1 scheme

SECURITY

Day3 App Domain Security

TLSv1.3 · HSTS · DMARC

SECURITY

Day3 App Vulnerability Disclosure

security.txt · contact published

SECURITY

Day3 App Trust Center

SOC 2, GDPR

SECURITY

Resources

Get Started 3

Portal, sign-up, and the first successful call

SignUp

SignUp

GettingStarted

GettingStarted

DeveloperPortal

DeveloperPortal

Documentation 1

Reference material describing how the API behaves

Documentation

Documentation

Agent Surfaces 2

MCP servers, agent skills, and machine-readable catalogs

LLMsTxt

LLMsTxt

WellKnown

WellKnown

Design & Contract 8

Pagination, idempotency, versioning, errors, and events

Spectral

JSONLD

Vocabulary

Webhooks

DataModel

Idempotency

Conventions

Conformance

Scroll for all 8

Access & Security 7

Authentication, authorization, and security posture

Compliance

Authentication

SecurityTxt

Security

TrustCenter

VulnerabilityDisclosure

VulnerabilityDisclosure

DomainSecurity

DomainSecurity

Scroll for all 7

Operate 2

Status, limits, changes, and where to get help

ChangeLog

ChangeLog

Support

Support

Commercial 4

Pricing, plans, and the legal terms of use

Plans

TermsOfService

PrivacyPolicy

Pricing

Company 2

The organization behind the API

Blog

Blog

Website

Website

Other 2

Properties that don't map to a standard resource type

Hosts

Hosts

Vendors

Vendors

Source (apis.yml)

aid: day3-app
name: day3
accessModel:
  pricing: paid
  onboarding: unknown
  trial: false
  try_now: false
  public: false
  label: Paid
  confidence: medium
  probe_state: unreadable
  source:
  - plans
  - authentication
  - security
  generated: '2026-11-06'
  method: derived
image: https://day3.app/opengraph-image?0c8c7c3940d0fd10
description: Day3 provides email marketing and transactional email services billed by the number of emails sent. It offers
  unlimited subscribers, tiered pricing starting at $1 per month, and features such as deliverability tools, GDPR compliance,
  and one‑click unsubscribe. The platform targets startups and indie developers looking for a simple, usage‑based pricing
  model.
url: https://artifacts.apis.io/day3-app/apis.yml
x-type: company
x-source: harvest:mcp-registry
x-tier: profiled
x-tier-reason: enrichment pass 2026-11-05 (modular-v1; was harvest)
specificationVersion: '0.20'
created: '2026-11-05'
modified: '2026-11-05'
tags:
- Email Marketing
- Transactional Email
- Software-as-a-Service
- Startup Tools
tags_raw:
- EmailMarketing
- TransactionalEmail
- SaaS
- StartupTools
- API
maintainers:
- FN: Kin Lane
  email: kin@apievangelist.com
- FN: APIs.json
  email: info@apis.io
common:
- type: Plans
  url: plans/day3-app-plans-pricing.yml
- type: Spectral
  url: rules/day3-app-rules.yml
- type: JSONLD
  url: json-ld/day3-app-context.jsonld
- type: Vocabulary
  url: vocabulary/day3-app-vocabulary.yml
- type: Webhooks
  url: asyncapi/day3-app-webhooks.yml
- type: DataModel
  url: data-model/day3-app-data-model.yml
- type: Idempotency
  url: conventions/day3-app-conventions.yml
- type: Conventions
  url: conventions/day3-app-conventions.yml
- type: Compliance
  url: https://day3.app/security
- type: Authentication
  url: authentication/day3-app-authentication.yml
- type: Conformance
  url: conformance/day3-app-conformance.yml
- type: LLMsTxt
  url: llms/day3-app-llms.txt
- type: SecurityTxt
  url: well-known/day3-app-security.txt
- type: WellKnown
  url: well-known/day3-app-well-known.yml
- type: Hosts
  url: hosts/day3-app-hosts.yml
- type: Vendors
  url: vendors/day3-app-vendors.yml
- type: TermsOfService
  url: https://day3.app/terms
- type: SignUp
  url: https://day3.app/features/signup-forms
- type: Security
  url: https://day3.app/security
- type: PrivacyPolicy
  url: https://day3.app/privacy
- type: ChangeLog
  url: https://day3.app/changelog
- type: Blog
  url: https://day3.app/blog
- type: GettingStarted
  url: https://docs.day3.app/quickstart
- type: DeveloperPortal
  url: https://docs.day3.app\
- type: TrustCenter
  url: security/day3-app-trust-center.yml
- type: VulnerabilityDisclosure
  url: security/day3-app-vulnerability-disclosure.yml
- type: DomainSecurity
  url: security/day3-app-domain-security.yml
- type: Website
  url: https://day3.app/
- type: Documentation
  url: https://docs.day3.app/
- type: Pricing
  url: https://day3.app/pricing
- type: Support
  url: mailto:hello@day3.app
apis:
- name: Day3 API
  description: Public API for Day3 email marketing and transactional email services. Documentation is available at the developer
    portal.
  humanURL: https://docs.day3.app/
  aid: day3-app:day3-api
- aid: day3-app:day3-app-api
  name: day3 API
  description: 'day3 API as documented publicly: 19 operations. Contract generated from the documentation by API Evangelist
    (2026-11-05); not

    the provider''s own document.'
  humanURL: https://day3.app/features/api
  baseURL: https://go.day3.app
  properties:
  - type: OpenAPI
    url: openapi/_ae-authored/day3-app-openapi-generated.yml
    x-provenance: generated
  - type: JSONSchema
    url: json-schema/day3-app-get-emails-id-response-schema.json
  - type: JSONSchema
    url: json-schema/day3-app-post-api-v1-emails-response-schema.json
  - type: JSONSchema
    url: json-schema/day3-app-get-v1-webhooks-webhookid-deliveries-response-schema.json
  - type: JSONSchema
    url: json-schema/day3-app-post-your-endpoint-request-schema.json
  - type: JSONSchema
    url: json-schema/day3-app-get-v1-webhooks-response-schema.json
  - type: JSONSchema
    url: json-schema/day3-app-get-v1-webhooks-webhookid-response-schema.json
x-coverage:
  state: unreadable
  reason: no-machine-readable-spec
  detail: No OpenAPI, AsyncAPI, GraphQL, or other contract was found despite documented API reference.
  evidence:
  - url: https://docs.day3.app/
    status: 200
  checked: '2026-11-05'
x-enrichment:
  date: '2026-11-05'
  status: enriched
  artifacts_added: 33
  pass: modular-v1
  engine: identity=local,contract=local,surfaces=local,docs=local,regulatory=local

day3 API — frequently asked questions

Does day3 have an API?

Yes. APIs.io lists 2 APIs from day3, including Day3 API and day3 API.

Is there an OpenAPI specification for the day3 API?

Yes. APIs.io indexes 1 OpenAPI specification for day3, each linked for download from this page.

How do I authenticate with the day3 API?

day3 documents its authentication (1 scheme). The details are profiled on APIs.io as Day3 App Authentication.

How much does the day3 API cost?

day3 publishes its prices; its 1k plan is $1/mo as of 2026-11-05. APIs.io lists 9 pricing plans for day3, with each plan's price, limits and inclusions under Pricing on this page.

Is the day3 API ready for AI agents?

day3 has a Kin Score of 51.8 out of 100 (developing) and an Agent Readiness score of 44.2 (agent-ready), scored by API Evangelist on 2026-11-06.

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