SMTP2GO API + MCP (2026): the specialist relay's REST v3, scored

SMTP2GO has been running a specialist SMTP relay since 2006, and the modern REST API is the deliberately-minimal HTTPS layer bolted on top of that relay. The v3 API at https://api.smtp2go.com/v3/ covers sending, scheduling, templates, suppressions, webhooks, subaccounts and stats through 9 official SDKs under the smtp2go-oss GitHub organisation. SMTP2GO ships a first-party MCP server at https://developers.smtp2go.com/mcp, a remote Streamable HTTP endpoint carried in the AI section of the developer docs, and three hosted community bridges (viaSocket, Composio, Zapier MCP) fill in for teams that want a native Claude / Cursor / ChatGPT connector today.

At a glance

MCP integration in 2026

Model Context Protocol lets an LLM agent (Claude Desktop, Cursor, ChatGPT connectors, custom autogen graphs) call SMTP2GO the way a human clicks buttons. SMTP2GO publishes its own remote server, and three third-party proxies wrap the same v3 REST endpoints listed further down this page. The difference between them is tool shape: the first-party server bridges the whole API surface, the proxies expose a curated action set.

Available MCP servers

The first-party server sits alongside three hosted community options. All are HTTPS proxies to api.smtp2go.com/v3/; the differences are pricing model, auth flow and how much extra plumbing they add.

SMTP2GO API essentials

The v3 surface is deliberately narrow compared with SendGrid’s sprawling Web API v3 or Mailgun’s 2018-era design: roughly 18 resource families, all under /v3/{resource}/{action}, all POST-first (a few PATCH). If you have used Postmark’s compact API, the shape will feel familiar, but SMTP2GO leans harder on a single “everything is POST with a JSON body” convention rather than resource-styled REST verbs.

Authentication methods

SMTP2GO supports two mechanisms for authenticating REST calls; there is no OAuth flow because the API is designed for server-to-server automation, not delegated third-party access.

API key in header (recommended)

Create a key under Settings › API Keys. Keys are prefixed api- followed by 32 hex characters. Send them in the X-Smtp2go-Api-Key HTTP header. Keys support per-endpoint scopes (e.g. “email/send only, no admin”), IP allow-list bindings, and a sandbox flag that accepts sends without delivering.

API key in JSON body (legacy)

The original v3 pattern accepted {"api_key":"api-XXXX…"} as a field inside the JSON body. It still works and every SDK will do it if asked, but it leaks the key into request/response logs, monitoring tools, and any 4xx error message that echoes the payload. Prefer the header form for anything past a proof-of-concept.

Rate limits

SMTP2GO publishes a small number of hard limits and configures the rest per API key. The dashboard shows current per-key usage in real time.

Per-endpoint limits other than /activity/search are not published; SMTP2GO expects you to negotiate them per key. When you hit a limit the API returns HTTP 429 with a Retry-After header. The batching workaround is straightforward: shift high-fanout jobs onto /email/batch (up to 1,000 messages per POST), and pin polling jobs against /activity/search to a single worker with exponential backoff. If you routinely burst above 200,000/hour, open a support ticket to lift the ceiling before your growth curve does it for you.

Official SDKs

SMTP2GO maintains 9 official libraries in the smtp2go-oss GitHub organisation. Coverage skews toward the languages actual paying customers use (Python, PHP, .NET, WordPress), with Rust included for the systems-programming crowd. All wrap the same v3 endpoints; the Django and WordPress packages plug into their frameworks’ native mail hooks instead of forcing manual client calls.

Notable community SDKs

The community layer is thin because official coverage is broad. Look for framework adapters rather than raw HTTP clients: Laravel projects wrap the PHP SDK in service providers, Symfony wires it into the Mailer transport, and Elixir teams typically call the v3 API directly through Req or Finch since there is no first-party BEAM SDK. If your stack is Deno, Bun or Cloudflare Workers, the Node SDK works with minor polyfills but the officially-blessed path is fetch() against the REST endpoints.

Endpoints reference

The full endpoint index lives at apidocs.smtp2go.com. The 18 resource families below cover everything except the rarely-touched account and billing surfaces. All paths hang off the /v3/ prefix; all use POST unless noted.

Code examples

Two canonical snippets: send an email with the official Python SDK, then send with the Node.js SDK. Both assume the API key lives in the environment. Swap api-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX for the real value only in local scripts, production code should always read from a secret manager.

Python: send a transactional email

from smtp2go.core import Smtp2goClient

client = Smtp2goClient(api_key='api-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX')
response = client.send(
    sender='hello@yourdomain.com',
    recipients=['user@example.com'],
    subject='Hello from SMTP2GO',
    text='Plain text body',
    html='<h1>Hi from SMTP2GO</h1>',
    custom_headers={'X-Campaign': 'welcome-v1'},
)
print(response.json)

Node.js: send with the official SDK

import SMTP2GOApi from 'smtp2go-nodejs';

const api = SMTP2GOApi(process.env.SMTP2GO_API_KEY);

const mail = api.mail()
  .from({ email: 'hello@yourdomain.com', name: 'Your App' })
  .to({ email: 'user@example.com', name: 'User' })
  .subject('Hello from SMTP2GO')
  .html('<h1>Hi from SMTP2GO</h1>')
  .text('Plain text body');

const res = await api.client().consume(mail);
console.log(res.success, res.data);

curl: schedule an email 24 hours ahead

curl -X POST https://api.smtp2go.com/v3/email/send 
  -H "X-Smtp2go-Api-Key: api-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" 
  -H "Content-Type: application/json" 
  -d '{
    "sender": "hello@yourdomain.com",
    "to": ["user@example.com"],
    "subject": "Scheduled hello",
    "html_body": "<p>Delivered tomorrow.</p>",
    "schedule": "2026-08-20T09:00:00Z"
  }'

Common gotchas

Deprecations and changelog

SMTP2GO does not publish a machine-readable changelog feed. The API reference and the product blog together carry the announcement flow. Notable 2026 events:

• September 15, 2026-The AI section of the developer docs publishes a first-party MCP server at https://developers.smtp2go.com/mcp, a remote Streamable HTTP endpoint with five tools. This page previously stated that no first-party server existed. SMTP2GO publishes no launch date for it; the docs page carries version 3.0.4.
• July 30, 2026-IP allow lists gained a type parameter so a rule can target SMTP-only or API-only traffic instead of both.
• June 29, 2026-Master-account API keys can now manage subaccount suppressions by passing subaccount_id to the suppression endpoints.
• June 11, 2026-Request-body size limits enforced: 1 MB for standard endpoints, 50 MB for email endpoints.
• June 3, 2026-Email scheduling up to 3 days ahead (schedule property + /email/scheduled/*), /email/batch for up to 1,000 messages per request, and a fastaccept parameter for higher-throughput sending.
• May 25, 2026-Developer docs added an AI/MCP section describing how AI agents should call the v3 REST API directly.

No endpoints have been formally deprecated in 2026. The v2 API remains reachable for legacy customers but new keys default to v3 and the docs no longer surface v2 examples. Follow the official release notes for breaking-change announcements.

Frequently asked questions