MailTester Email Verification

What the MailTester Email Verification MCP server does

The MailTester Email Verification MCP server gives MCP-compatible AI agents access to MailTester’s email checking service. It is designed to assess an address before an application or workflow sends mail to it. Results use four categories: valid, invalid, risky, and unknown.

A valid result means the mailbox appears to exist and accept mail. Invalid indicates that the address or its domain cannot receive mail. Risky covers cases such as catch-all domains, role accounts, disposable addresses, and some Microsoft 365 addresses that cannot be confirmed by the receiving server. Unknown means the receiving server did not provide a conclusive response.

How it works

The service is hosted at https://app.mailtester.com/mcp and communicates through Streamable HTTP with JSON responses. There is no local package or process to install. An agent connects to the endpoint, supplies a bearer key when required, and invokes the verification tools through MCP.

The single-address operation uses one credit. Batch verification accepts up to 25 addresses in one call and returns a result for each address. The same key can also be used with MailTester’s REST endpoints for individual verification, batch verification, and credit lookup.

Setup and configuration

The MailTester Email Verification MCP server can issue a free agent key through the get_api_key tool without requiring an existing key. Alternatively, a client can register through the agent gateway. The returned key is displayed once, expires after 30 days, and includes 25 free verifications per day; unused daily credits do not carry over.

Configure the remote HTTP endpoint in an MCP client and send the key as an Authorization: Bearer YOUR_KEY header. The documented connection examples cover Claude Code, Claude Desktop, Cursor, and VS Code. The endpoint can also be connected using the service’s agent documentation, OpenAPI description, or agent card.

Tools and capabilities

Available tools include:

• get_api_key to obtain a free key.
• verify_email to check one address and consume one credit.
• verify_batch to check as many as 25 addresses in one request.
• explain_result to describe a verdict in plain language and provide a send recommendation.
• get_batch_status, list_lists, and get_list to follow saved verification lists.
• estimate_cost and get_credit_balance to inspect expected usage and remaining credits.

The checks can identify domain and SMTP conditions, catch-all behavior, and disposable addresses. The result explanation tool is useful when an agent needs to turn a categorical result into an operational recommendation.

Limitations and notes

The MailTester Email Verification MCP server limits each key to 10 requests per minute and permits only one request in flight per key. Free agent keys also share a daily capacity; once that capacity is reached, requests return HTTP 429 until 00:00 UTC. These limits matter when processing lists concurrently or running automated signup checks.

The service does not provide inbox placement, blacklist monitoring, or email-finder functionality through the free agent access described here. Those capabilities require a full account at MailTester. An unknown result is not confirmation that delivery will fail; it means the receiving server did not give a definite answer. A risky result likewise indicates conditions that warrant review rather than a guaranteed delivery outcome.