MailTester MCP server — email verification for AI agents

Remote Model Context Protocol server that tells an AI agent whether an email address can receive mail before anything is sent to it.

• Endpoint: https://app.mailtester.com/mcp (Streamable HTTP, JSON responses)
• Official MCP Registry: com.mailtester/email-verification
• Website: https://mailtester.com
• Agent docs: https://app.mailtester.com/agent-gateway · OpenAPI · llms.txt · agent card

The server is hosted; there is nothing to install. This repository documents how to connect.

Get a key (no account needed)

Connect without a key and call the get_api_key tool, or:

curl -X POST https://app.mailtester.com/agent-gateway/register \
  -H 'Content-Type: application/json' \
  -d '{"name":"my-agent","purpose":"verify signup emails"}'

The key is shown once. Each key gets 25 free verifications per day (unused credits do not roll over) and expires after 30 days; register again when it does. Send it as Authorization: Bearer <api_key>.

Connect

Claude Code

claude mcp add --transport http mailtester https://app.mailtester.com/mcp --header "Authorization: Bearer YOUR_KEY"

Claude Desktop / Cursor / VS Code (mcp.json-style config)

{
  "mcpServers": {
    "mailtester": {
      "type": "http",
      "url": "https://app.mailtester.com/mcp",
      "headers": { "Authorization": "Bearer YOUR_KEY" }
    }
  }
}

Tools

Tool	What it does
get_api_key	Get a free key (works without a key)
verify_email	Verify one address → valid, invalid, risky or unknown (1 credit)
verify_batch	Verify up to 25 addresses per call, with per-address verdicts
get_batch_status, list_lists, get_list	Follow saved verification lists
explain_result	Explain a verdict in plain English, with a send recommendation
estimate_cost, get_credit_balance	Check credits before running

Verdicts

• valid: the mailbox exists and accepts mail
• invalid: the address or domain cannot receive mail
• risky: accept-all (catch-all) domain, role or disposable address, or a Microsoft 365 address the mail server couldn't confirm
• unknown: the receiving server gave no definitive answer

Limits

10 requests per minute per key, one request in flight per key, and a shared daily capacity for free agent keys (HTTP 429 until 00:00 UTC when it is used up). Inbox placement, blacklist monitoring and email finder need a full account at https://mailtester.com.

REST API

The same key works on the REST API: POST https://app.mailtester.com/api/v1/verify, POST https://app.mailtester.com/api/v1/verify/batch, GET https://app.mailtester.com/api/v1/credits. See the OpenAPI description.