Ready APIs

Email SMTP RCPT Probe

email_smtp_probe

Read-onlyIdempotent

Live SMTP RCPT probe (HELO + MAIL FROM + RCPT TO; no DATA). Reports deliverability verdict (deliverable / catch_all / undeliverable / greylisted / inconclusive). Catch-all detection via random-fake-RCPT comparison. Skipped automatically for Gmail / Outlook / Yahoo / iCloud / Fastmail / ProtonMail. ~3-8s latency.

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_smtp_probe"
  }
}

Input Schema

Name	Required	Description	Default
`email`	Yes	Email address to probe.
`check_catch_all`	No

Schema Changelog

Changes observed during successful MCP inspections.

First observedAug 14, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Annotations already mark readOnly/idempotent/nondestructive, and the description adds materially: no DATA, catch-all detection via random-fake-RCPT comparison, automatic skip for specified providers, and ~3-8s latency. This gives the agent useful behavioral expectations beyond the annotations.

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

Conciseness5/5

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

Four compact sentences, each carrying distinct information: mechanism, output, catch-all method, provider exemptions, and latency. Core action and output are front-loaded with no filler.

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?

With no output schema, the description still enumerates the possible verdicts, explains the probing protocol, discloses provider skips, and gives latency. That is sufficient for an agent to select and invoke the tool correctly.

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 documents only email; check_catch_all is a bare boolean. The description adds meaning to catch-all behavior via 'random-fake-RCPT comparison' and the auto-skip caveat, but it never explicitly names check_catch_all or states that it can be disabled, so the compensation is partial.

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

Purpose5/5

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

States a specific verb and resource: 'Live SMTP RCPT probe (HELO + MAIL FROM + RCPT TO; no DATA)' with a concrete output enum. The protocol detail and verdict list distinguish it from sibling email/address validation tools even without naming an alternative.

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?

The description clearly implies use for live SMTP deliverability checking, includes catch-all detection, and notes automatic skipping for major providers. It does not explicitly name alternatives or state when not to use it, so it has strong contextual guidance but no formal exclusions.

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

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