Email Preflight

email_preflight

Use before sending or processing an email when you need current preflight facts about the address. For $0.001 USDC on Base mainnet, the existing Email Preflight API checks six facts: syntax validity, domain availability, MX availability, disposable-email detection, role-account detection, and free-provider detection. Example paid result shape: { email: "user@example.com", syntax: "valid", domain: "exists", mx: "found", disposable: "not_listed", role_based: "not_detected", free_provider: "not_detected" }. MX records do not prove that a mailbox exists. Unpaid requests return the original upstream HTTP 402 challenge. A compatible x402 client may retry with its own signed PaymentPayload in params._meta["x402/payment"]. This adapter never signs, owns a wallet, or settles independently.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "email": "<email>"
    },
    "name": "email_preflight"
  }
}

Input Schema

Name	Required	Description	Default
`email`	Yes	Email address to check.

Schema Changelog

Changes observed during successful MCP inspections.

First observedOct 16, 2026

TDQS

A4.3/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full burden and does so: it discloses the $0.001 USDC Base mainnet cost, the six checked facts, an example result shape, the crucial limitation that 'MX records do not prove that a mailbox exists,' the 402 behavior on unpaid requests, the x402 PaymentPayload retry channel, and a non-custodial disclaimer. That is unusually complete behavioral disclosure for a paid API adapter.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Sentences are dense and front-loaded: usage context first, then price and the checked facts, then the example, then the MX caveat and the payment mechanics. Every sentence carries information, though the block is long enough that a reader must work through all of it to reach the payment instructions.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a one-parameter paid adapter with no annotations and no output schema, the description supplies the missing return shape via an example, the cost, the failure-mode behavior (402), and the recovery path (signed PaymentPayload). Nothing an agent needs in order to invoke it correctly is absent.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and there is only one declared parameter, so the baseline is 3, but the description adds meaning beyond the schema by documenting the out-of-band params._meta['x402/payment'] payment channel that an agent must use to get a paid result. The 'additionalProperties: false' schema does not advertise that channel, so the description earns extra credit for surfacing it.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource ('preflight facts about the address') and enumerates exactly the six facts checked (syntax, domain, MX, disposable, role-account, free-provider), so the agent knows precisely what the tool returns. It never differentiates itself from the siblings email_check_mini or url_preflight, which is the only thing keeping it from a 5.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

'Use before sending or processing an email' gives a clear triggering context, and the payment/402 section explains what happens on an unpaid call. It stops short of naming when NOT to use it or pointing to email_check_mini as the alternative, so it is clear context without explicit exclusions.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

All Email Preflight tools Inspect

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