Email API for AI agents

Name: Email API for AI Agents: MCP Server and Sandbox | UniOne
Brand: UniOne

Give Claude Code, Cursor, Codex or any other AI agent what it needs to add transactional email to your app: a hosted MCP server, a REST API and a sandbox domain that sends your first test email before you touch DNS.

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Quick start

Your first test email in minutes, no DNS setup

The sandbox is for testing: up to 10 confirmed recipients and 100 emails a day. To email your users, verify your own domain.

Connect your agent

Add the UniOne MCP server and sign in to UniOne in your browser.

Your agent gets a sandbox domain

It's a test domain that comes with every account, with DKIM and SPF already set up.

Confirm your inbox

UniOne sends a confirmation link to your address. Click it, and the sandbox can send to that address.

Get the test email

Your agent sends it from the sandbox domain and shows you the job_id.

Setup prompt

Setup prompt for AI coding agents

Paste this prompt into Claude Code, Cursor, Codex or any other coding agent. It reads the UniOne docs, sends a test email from the sandbox domain and adds sending to your app's server code. When you're ready for production, it walks you through verifying your domain.

Docs your agent reads: llms.txt · llms-full.txt · OpenAPI spec · MCP server docs · pricing.md

API key

Calling UniOne

First email

Production

Code quality

API key

Integrate UniOne transactional email into this project. Follow these rules exactly.

1. API key
- Application code reads the UniOne API key from the environment variable UNIONE_API_KEY and calls UniOne only server-side: an API route, server function, edge function or backend worker.
- Never ask me to paste the key into chat, never hard-code or commit it, and never use it in browser or client-side code.
- The UniOne MCP server does not use UNIONE_API_KEY. If it is connected, use its tools even when the variable is not set.
- If UNIONE_API_KEY is not set, tell me to copy my key from https://cp.unione.io/en/settings/security/api into my environment or secrets manager. Don't run application code that calls UniOne until the key is set. If the MCP server is not connected either, stop and wait for the key.

Calling UniOne

2. How to call UniOne
- If the UniOne MCP server is connected, use its tools for setup and checks.
- In application code use the Web API: POST https://api.unione.io/en/transactional/api/v1/<method>.json with a JSON body and the X-API-KEY header. Do not send api_key in the body.
- Docs for you: https://docs.unione.io/llms.txt and the OpenAPI spec https://docs.unione.io/openapi/unione-web-api-latest.json. Do not invent methods or fields that are not in the spec.

First email

3. First email: sandbox domain, no DNS
- Get the account's sandbox domain: get_sandbox_domain (MCP) or domain/get-sandbox-domain (REST). This and the confirmed-email methods don't work with a project key: use the MCP server connected by sign-in or the account API key.
- The sandbox domain delivers only to confirmed addresses. Ask me which inbox to use. If it is not confirmed yet, add it (add_confirmed_email (MCP) or confirmed-email/create (REST)) and wait until I click the link in the confirmation email (valid for 48 hours).
- Send one test email from noreply@<sandbox-domain> to that address and show me the job_id.

Production

4. Before production
- The sandbox is test-only: confirmed recipients only, 100 emails a day, the domain expires after 31 days. Never ship a sandbox domain as the production sender; read from_email from config.
- Help me verify my own domain: domain/get-dns-records → add the records to DNS → domain/validate-verification-record and domain/validate-dkim → domain/list until verification is confirmed and DKIM is active.

Code quality

5. Code quality
- Set message.idempotence_key on sends that can be retried.
- Max 500 recipients and 10 MB per request; always include body.html or body.plaintext.
- Get delivery events with a webhook (webhook/set) and a fast, idempotent HTTPS handler instead of polling.
- Keep default unsubscribe and tracking behavior; changing it requires UniOne support.
- Ask me before any action that sends real email or deletes anything.

Trusted by Industry Leaders

MCP

Connect the UniOne MCP server

Let your agent handle email in plain language: send messages, get the sandbox domain, set up your own domain, build templates, wire up webhooks, check the suppression list and validate addresses. The server is hosted, so there is nothing to install.

Server URL: https://mcp.unione.io/mcp

Add the URL to your agent and sign in when it asks: your browser opens UniOne, you click Allow access, and you're connected. There is no API key to copy.

claude mcp add --transport http unione https://mcp.unione.io/mcp
# then run /mcp, select unione and choose Authenticate

codex mcp add unione --url https://mcp.unione.io/mcp
codex mcp login unione

// ~/.cursor/mcp.json, then click Connect
{
  "mcpServers": {
    "unione": { "url": "https://mcp.unione.io/mcp" }
  }
}

// .vscode/mcp.json, then click Start
{
  "servers": {
    "unione": { "type": "http", "url": "https://mcp.unione.io/mcp" }
  }
}

URL:        https://mcp.unione.io/mcp
Transport:  Streamable HTTP
Sign-in:    OAuth, in your browser (UniOne account)

Setup for every client, including Windsurf, Claude and ChatGPT, and all 32 tools: UniOne MCP server docs.
Ready-made requests to try and what your agent can do: Send email with MCP.

Tools that send or delete are marked destructive, so your client asks you to confirm first. Revoke access at any time in Settings → Security → Connected applications.

Coding agents

Send email from Claude Code, Cursor and other coding agents

Claude Code

Add the UniOne MCP server with one command and send email from your terminal. Setup →

Cursor

Add UniOne to mcp.json and let Cursor send email and set up your domain. Setup →

Codex

Connect UniOne with codex mcp add and sign in from the terminal. Setup →

GitHub Copilot

Add UniOne to VS Code and use it in Copilot agent mode. Setup →

Windsurf

Add UniOne to the Windsurf MCP config and send email from Cascade. Setup →

Gemini CLI

Add UniOne with gemini mcp add and sign in from the terminal. Setup →

ChatGPT

Turn on developer mode and paste the server URL as a new connector. Setup →

Any MCP client

Any client that supports Streamable HTTP connects the same way: add the URL and sign in to UniOne. Setup →

Any agent without MCP

Paste the setup prompt, and your agent calls the REST API from your server. Copy the prompt →

Customer Testimonials

Denis K.

January 28, 2025

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Deep K.

November 23, 2024

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Alex B.

August 29, 2024

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Ivan M.

August 1, 2024

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Deep K.

November 23, 2024

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Ivan M.

August 1, 2024

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Why UniOne

Why UniOne for AI-built apps

Test email without DNS

Every account has a sandbox domain with DKIM and SPF already set up. Your agent sends a real test email before you touch DNS records.

Data centers in the EU and the US

Your account is created in the EU or US data center, depending on where you sign up. One API address, api.unione.io, routes every call to your account's region. No regional URLs in your code.

Free plan, paid plans from $6

The Free plan gives you 100 emails a day with no credit card, to addresses on your own domain or to confirmed addresses. Ready to email customers? The 4-month free trial gives you 6,000 emails a month, then $4 a month. Paid plans start at $6 for 10,000 emails.

Safe for agents by default

Keys stay on the server, the sandbox stays in testing, and your client asks you to confirm before the agent sends or deletes anything. The setup prompt gives your agent the rules below.

Book consultation

Rules

Email integration rules for AI agents

Your agent gets these rules from the setup prompt. They apply to the code your app runs in production: use them to review what the agent writes.

Key on the server

Your app sends email from server code. The API key lives in an environment variable and goes in the X-API-KEY header, never in the browser.

Sandbox for testing

The sandbox is for testing. In production, send from your own verified domain with active DKIM.

Idempotent sends

Set idempotence_key on every send that can be retried.

Request limits

Up to 500 recipients and 10 MB per request.

Webhooks for events

Get delivery events with webhooks. Keep the handler fast and idempotent.

Default unsubscribe and tracking

Stay on the default path: changes to unsubscribe and tracking go through support.

FAQ

What is the UniOne MCP server and how do I set it up?

The UniOne MCP server is a hosted Model Context Protocol server at https://mcp.unione.io/mcp. It gives AI agents 32 tools to send email, manage domains, templates and webhooks, and check addresses. Add the URL to your agent and sign in to UniOne in your browser. There is nothing to install. See the MCP server docs.

Is UniOne free for AI agent projects?

Yes, you can start on the Free plan: 100 emails a day and 5 email validations a month, with no credit card and no time limit. On it, you send from your verified domain or the sandbox domain to addresses on your own domain or to confirmed addresses. For more volume, the free trial gives you 6,000 emails and 100 email validations a month for 4 months. It needs a card, and after the trial the plan continues at $4 a month. Paid plans start at $6 for 10,000 emails. The MCP server costs nothing extra: you pay for emails and validations, as with the API.

How do I send email from Codex, Claude Code or Cursor?

Add the UniOne MCP server. In Codex, run codex mcp add unione --url https://mcp.unione.io/mcp, then codex mcp login unione. In Claude Code, run claude mcp add --transport http unione https://mcp.unione.io/mcp, then /mcp → Authenticate. In Cursor, add the server URL to ~/.cursor/mcp.json and click Connect. Then ask the agent to send a test email. To add sending to your app's code, paste the setup prompt above.

Can my agent send a test email without setting up DNS?

Yes. Every account has a sandbox domain with DKIM and SPF already set up. Your agent gets it, adds your address and sends the test email after you click the link in the confirmation email. The sandbox sends only to confirmed addresses, up to 10, and up to 100 emails a day. To email your users, verify your own domain. Your agent can walk you through the DNS records.

Is it safe to connect an AI agent to my email account?

Every MCP tool is marked read, write or destructive, so your client asks you to confirm before the agent sends or deletes anything. When you connect by signing in, the agent never sees your password or API keys, and you can revoke access at any time in Settings → Security → Connected applications. Keys and tokens are never written to our logs.

Do I need MCP to send email from an AI agent?

No. MCP lets the agent set things up for you: the sandbox, your domain, templates and webhooks. Your application code calls the REST API directly with the key kept on the server. The setup prompt covers both.

How do I add email to a Lovable, Bolt, Replit or v0 app?

Paste the setup prompt into the platform's chat. UniOne calls run in a server-side function, so the key never reaches the browser: a Supabase Edge Function in Lovable, a server route in Bolt, a backend call with Replit Secrets in Replit, a Next.js Route Handler in v0.

Which AI agents work with UniOne?

Any MCP client that supports Streamable HTTP. Setup steps are ready for Claude Code, Cursor, Codex, GitHub Copilot in VS Code and Windsurf, and for Claude and ChatGPT, where you add the server URL as a custom connector. Without MCP, any agent can use the REST API with the setup prompt above.

Send your first test email in minutes

Start on the Free plan, with no credit card: 100 emails a day to addresses on your own domain or to confirmed addresses. Your agent sends the first test email from the sandbox domain before you set up DNS. When you need more volume, choose a paid plan or talk to our sales team about a custom plan.

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