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How to connect Mailcheck to Claude

Connect Claude Web, Desktop and Cowork to Mailcheck MCP. Check if this email address is valid, verify a list of customer emails, and more using natural language, with authentication handled for you.

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Mailcheck is an email verification service that checks email validity in real time. It helps businesses reduce bounce rates and improve email deliverability.

2 Tools

Introduction

Claude is Anthropic's AI assistant, available on the web, in the desktop app, and on mobile. Cowork is its agent for knowledge work, and it runs on all of those surfaces too. Connected to your apps, Claude can work with your files and services to accomplish complex tasks on your behalf.

This guide walks you through the easiest and most secure way to connect your Mailcheck account to Claude via Composio Connect, enabling it to check if this email address is valid, verify a list of customer emails, test deliverability for a new subscriber, and more such actions on your behalf without compromising your account security.

Setup is the same on Claude Web, Desktop, and Cowork, and you only need to do it once. The connector is tied to your account, so it's available in all three.

Also integrate Mailcheck with

Claude CodeOpenAI Agents SDKClaude Agent SDKCodexOpenClawHermesAtomic AgentGoogle ADKLangChainVercel AI SDKMastra AILlamaIndexCrewAIPydantic AIAutoGen

Connecting Mailcheck to Claude Web, Desktop, and Cowork

Two ways to add the Composio connector. Pick whichever you prefer.

1. Add with one click

Click the button. Claude opens with the Composio connector filled in. Click Add, then go to step 2.

Add to Claude

Add manually

In Claude Desktop, click Customize in the left sidebar, then select Connectors and click the + icon at the top.

Claude Desktop connectors screen with Add custom connector selected

Click Add custom connector and paste in the Composio MCP server URL:

bash
https://connect.composio.dev/mcp
Add custom connector dialog with Composio MCP server URL

2. Authorize in your browser

Click Connect. You'll be redirected to a browser window where you can authorize Composio to continue.

Composio authorization screen for Claude Web, Desktop, and Cowork

3. Connect your Mailcheck account

Back in Claude, ask it to connect to Mailcheck or give it any Mailcheck-related task.

For example, ask Claude to:

  • "Check if this email address is valid"
  • "Verify a list of customer emails"
  • "Test deliverability for a new subscriber"

It will prompt you to authenticate and authorize access.

That's it. Composio's tools are now available in Claude, and your Mailcheck account is ready to use.

What is the Mailcheck Claude Connector, and what's possible with it?

The Mailcheck MCP server is an implementation of the Model Context Protocol that connects your AI agent and assistants like Claude, Cursor, etc directly to your Mailcheck account. It provides structured and secure access to email verification tools, so your agent can validate email addresses, ensure email deliverability, and reduce bounce rates automatically on your behalf.

  • Real-time email verification: Instantly check if a provided email address is valid, reducing the risk of sending to invalid contacts.
  • Bulk verification support: Allow your agent to verify multiple email addresses in one go, streamlining large-scale list cleaning.
  • Deliverability assurance: Ensure that your outbound emails reach real inboxes by validating addresses before sending campaigns or notifications.
  • Fraud and spam prevention: Detect and filter out suspicious or disposable email addresses to protect your business from fraudulent signups or spam accounts.

Troubleshooting

1. Why does Claude not use the Composio connector for Mailcheck?

The connector can show as connected, but Claude must use it in each Mailcheck conversation.

  • Click + at the lower left of the chat, or type /.
  • Hover Connectors.
  • Turn on Composio.
  • Ask Claude to connect to Mailcheck again.

2. Why does Claude show an MCP server error before Mailcheck connects?

Claude can show Couldn't reach the MCP server or Authorization with the MCP server failed when you add the connector for Mailcheck.

  • Check that the connector URL is https://connect.composio.dev/mcp.
  • Connectors run from Anthropic's cloud, so your device network is usually not the problem.
  • Add the connector again from Customize > Connectors, then ask Claude to connect Mailcheck.
  • If it fails again, send the error and the ofid_ reference id to support@composio.dev.

3. Why does the Mailcheck authorization link not work?

The Composio connector is one server. Your Mailcheck account connects separately when Claude asks for it.

  • If the browser does not open, or the link expires, ask Claude to retry the Mailcheck connection.
  • Composio creates a fresh Mailcheck authorization link.
  • Open that link in your browser and finish the Mailcheck sign-in.

4. Why do Mailcheck actions fail after they worked before?

The Mailcheck connection can expire or lose permission.

  • Ask Claude to inspect the Mailcheck connection.
  • If the connection is unhealthy, disconnect it when Claude prompts you.
  • Reconnect the Mailcheck account and retry the action.
  • If Claude shows a permission prompt, approve it before Claude changes data in Mailcheck.
  • If the action still fails with a permission error, check that the connected account has that permission in Mailcheck itself.

5. How do I switch to a different Mailcheck account?

Claude can manage the Mailcheck connection for you.

  • Ask Claude to disconnect or delete the current Mailcheck account.
  • Ask Claude to connect Mailcheck again.
  • Open the new Composio authorization link for Mailcheck.
  • Sign in with the other Mailcheck account in the browser.
  • After the account connects, ask Claude to retry the Mailcheck task.

6. Why is my Mailcheck connection missing in Claude Desktop or on a Team plan?

The connector belongs to your Claude account, so your Mailcheck connection works in Claude Web, Desktop, and Cowork.

  • In Claude Desktop, open Customize > Connectors.
  • Next to Composio, click ⋮ and click Clear cache.
  • If Mailcheck tools still do not appear, click ⋮, disconnect, remove the connector, and add it again.
  • On Team or Enterprise, an Owner or Primary Owner must enable the connector before you can connect Mailcheck.
TOOLS

Supported Tools

Every Mailcheck action and event your agent gets out of the box.

Validate Domain

Tool to validate a domain for disposability, MX records, domain age, and spam indicators.

Verify Email Address

Tool to verify provided email address.

FRAMEWORKS

How to build Mailcheck MCP Agent with another framework

Claude Code logo

Claude Code

Use Mailcheck MCP with Claude Code

OpenAI Agents SDK logo

OpenAI Agents SDK

Use Mailcheck MCP with OpenAI Agents SDK

Claude Agent SDK logo

Claude Agent SDK

Use Mailcheck MCP with Claude Agent SDK

Codex logo

Codex

Use Mailcheck MCP with Codex

OpenClaw logo

OpenClaw

Use Mailcheck MCP with OpenClaw

Hermes logo

Hermes

Use Mailcheck MCP with Hermes

Atomic Agent logo

Atomic Agent

Use Mailcheck MCP with Atomic Agent

Google ADK logo

Google ADK

Use Mailcheck MCP with Google ADK

LangChain logo

LangChain

Use Mailcheck MCP with LangChain

Vercel AI SDK logo

Vercel AI SDK

Use Mailcheck MCP with Vercel AI SDK

Mastra AI logo

Mastra AI

Use Mailcheck MCP with Mastra AI

LlamaIndex logo

LlamaIndex

Use Mailcheck MCP with LlamaIndex

CrewAI logo

CrewAI

Use Mailcheck MCP with CrewAI

Pydantic AI logo

Pydantic AI

Use Mailcheck MCP with Pydantic AI

AutoGen logo

AutoGen

Use Mailcheck MCP with AutoGen

FAQ

Frequently asked questions

With a standalone Mailcheck MCP server, the agents and LLMs can only access a fixed set of Mailcheck tools tied to that server. However, with the Composio Tool Router, agents can dynamically load tools from Mailcheck and many other apps based on the task at hand, all through a single MCP endpoint.

Yes, you can. Claude Cowork fully supports MCP integration. You get structured tool calling, message history handling, and model orchestration while Tool Router takes care of discovering and serving the right Mailcheck tools.

Yes, absolutely. You can configure which Mailcheck scopes and actions are allowed when connecting your account to Composio. You can also bring your own OAuth credentials or API configuration so you keep full control over what the agent can do.

All sensitive data such as tokens, keys, and configuration is fully encrypted at rest and in transit. Composio is SOC 2 Type 2 compliant and follows strict security practices so your Mailcheck data and credentials are handled as safely as possible.

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