SendX API + MCP (2026): REST-only v1, six SDKs, no MCP server

SendX exposes one REST surface at https://api.sendx.io/api/v1/rest, authenticated with a single header and backed by 6 official SDKs regenerated in late 2025. It is a small API by ESP standards, and that narrowness matters: there is no SMTP relay to fall back on, so every byte of programmatic mail goes through it. On the agent side the answer is blunter — as of August 2026 there is no official SendX MCP server and no community server in any major registry.

At a glance

MCP integration in 2026

The question in 2026 is no longer whether a vendor has an API but whether an agent can drive it on day one against a server someone else maintains. For SendX the answer is no — and knowing what sits in the SendX GitHub org matters, because two repositories there look like MCP servers and are not.

Available MCP servers

Two entries are real routes to agent access; two are SendX repositories often mistaken for MCP servers, listed so you can rule them out.

SendX API essentials

Everything lives under one base URL: no regional variants, no GraphQL surface, no async job API. Resources are singular nouns, responses are JSON for successes and errors alike, status codes are conventional. This is the whole contract.

The surface splits into three jobs. Audience management — contacts, lists, tags, custom fields, segments — is the bulk of it. Campaign management mirrors the dashboard closely enough that programmatic sends are viable. Behavioural I/O covers events, visitor identification and webhooks. The transactional endpoint sits apart as the only part gated behind a paid add-on and metered separately. No request timeout is documented, so set your own.

Authentication methods

One mechanism, one static key, no OAuth flow and no refresh token. First integration is fast; key hygiene is entirely yours.

Team API key in the X-Team-ApiKey header

Every call carries the key in a header named X-Team-ApiKey — not Authorization, not a bearer token, not a query parameter. Generate it at app.sendx.io/setting/connectors/api. It is team-scoped rather than seat-scoped and never expires on its own, so rotation is a manual operation you must schedule.

curl -X GET "https://api.sendx.io/api/v1/rest/contact?offset=0&limit=30" \
  -H "X-Team-ApiKey: $SENDX_API_KEY" \
  -H "Content-Type: application/json"

The transactional add-on key is a separate credential

The key used by POST /send/email is issued inside the add-on’s own panel — profile icon › Settings › Add-ons › Transactional Email › Settings — not the general connectors screen. Both travel in the same header, which is exactly why the failure confuses people: the request shape is identical, and the only symptom is an authorisation error on the send endpoint while every contact call keeps working.

Rate limits

SendX publishes no numeric rate limits anywhere in official documentation — only the line “Respect API rate limits for optimal performance”, with no figure attached. Every cell below that would normally hold a number is marked unpublished rather than filled from a comparison site.

The workaround is engineering discipline, not a documented allowance. With no bulk endpoint a 50,000-contact reconciliation is 50,000 requests, and with no published ceiling you cannot size concurrency in advance. Build the sync as a queue with a small worker pool — start at 2 to 4 concurrent requests — use exponential backoff with jitter, and checkpoint so an interrupted run resumes. Note too that pagination is contradictory in the docs: limit=30 is the only value both the prose and the OpenAPI schema accept, and the default of 10 silently truncates any loop that omits the parameter. If you need volume under published limits, pair SendX campaigns with a dedicated relay such as Brevo or Sender for app mail.

Official SDKs

All 6 clients are MIT licensed and generated from one OpenAPI description, so class names line up across languages (ContactApi, CampaignApi, EmailSendingApi, WebhookApi, TrackingApi). Five were regenerated the same day in September 2025; JavaScript was refreshed again that December.

Notable community SDKs

There is no meaningful third-party SDK ecosystem, and with six generated clients there is little reason for one. Outside those six — C#, Rust, Elixir, Kotlin — generate from the OpenAPI description or call the endpoints directly; a hand-rolled client is realistically 100 lines. Note that go-guerrilla and smtpd in the SendX org are SMTP server building blocks, not API clients and not a relay you can authenticate against.

Endpoints reference

The full reference is at docs.sendx.io/api-reference/introduction. Below are the 15 resources you will actually use, relative to the base URL. Collections accept offset, limit and search; single-resource paths take the opaque encrypted ID.

Beyond these, the API exposes /member for team administration, /report for campaign analytics, and /post, /post-category and /post-tag behind SendX’s blog features. One caveat: only POST /send/email and GET /contact are documented verbatim with full parameter and response detail. The rest follow the singular-noun convention and match Api class names in the official SDKs, but confirm them against the live reference before hard-coding.

Code examples

Two snippets cover the shapes you will write most: creating a contact through an official SDK, and sending transactional mail over raw REST. Both use the same header name, but the transactional key is issued separately.

Python — create a contact with the official SDK

# pip install sendx
import os
import sendx_python_sdk
from sendx_python_sdk.rest import ApiException

configuration = sendx_python_sdk.Configuration()
configuration.api_key['TeamApiKey'] = os.environ["SENDX_API_KEY"]

with sendx_python_sdk.ApiClient(configuration) as api_client:
    contact_api = sendx_python_sdk.ContactApi(api_client)
    contact_request = sendx_python_sdk.ContactRequest()
    contact_request.email = "johndoe@sendx.io"
    contact_request.first_name = "John"
    try:
        contact = contact_api.create_contact(contact_request)
        print("Contact created:", contact)   # id is a string, never an int
    except ApiException as e:
        print("SendX API error:", e.status, e.body)

Node.js — send a transactional email over REST

// Paid Transactional Email add-on required, plus a verified sending domain.
// NOTE: SendX has NO SMTP relay - transactional mail is REST only.
const res = await fetch('https://api.sendx.io/api/v1/rest/send/email', {
  method: 'POST',
  headers: {
    'X-Team-ApiKey': process.env.SENDX_TRANSACTIONAL_API_KEY,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    from: { email: 'hello@yourdomain.com', name: 'Your App' },
    to: [{ email: 'customer@example.com', name: 'Customer' }],
    replyTo: { email: 'support@yourdomain.com' },
    subject: 'Your receipt',
    htmlBody: '<h1>Thanks!</h1><p>Your order is confirmed.</p>',
    textBody: 'Thanks! Your order is confirmed.',
  }),
});

console.log(res.status, await res.json());
// Expected: 202 { status: 'success', message: 'Email queued for delivery' }

Common gotchas

Deprecations and changelog

SendX publishes no dated first-party changelog or release-notes feed, so the timeline below is reconstructed from timestamps on official repositories and the current documented state of official pages. Treat SDK dates as last-updated, not release dates.

• September 8, 2025 — a regenerated wave of official SDKs shipped across five languages at once (PHP, Java, Ruby, Python, Go), retiring the sendx-api-<lang> clients dormant since 2016–2020.
• December 23, 2025 — sendx-javascript-sdk received the newest update of the current generation, making it the de facto reference implementation for v1.
• April 3, 2026 — the SendX org updated openclaw, a TypeScript personal AI assistant project: agent investment, but explicitly not an MCP server for the SendX API.
• April 16, 2026 — email-skills updated: deliverability and compliance guidance packaged for AI-agent consumption.
• August 19, 2026 — state of record: Transactional Email add-on live as a REST-only, paid-plan-gated feature at $10/month for 3,000 emails plus $0.003 overage; marketing plans from $9.99/month for 1,000 contacts; still no SMTP relay and no official MCP server.

Track changes at the official API reference and the SendX repository list.

Frequently asked questions