RunStack Checks

Email domain check

check_email_domain

Read-onlyIdempotent

Audit a mail domain as other mail servers see it: MX, SPF (including the 10-DNS-lookup limit), DKIM, DMARC policy, PTR / forward-confirmed reverse DNS, SMTP STARTTLS on the MX hosts, MTA-STS, TLS-RPT, BIMI. Use when mail lands in spam or bounces, after changing mail provider or DNS, or before tightening DMARC. Use this (not check_dns) for any email deliverability or authentication question. Returns a verdict, findings with cause and fix, and a report link.

Input Schema

Name	Required	Description	Default
`domain`	Yes	Mail domain (the part after @), e.g. example.com
`dkimSelector`	No	DKIM selector to check in addition to common ones, e.g. google, selector1, mail. Optional.

Output Schema

Name	Required	Description
`ok`	Yes	false when the check could not run (see error / message)
`tool`	No
`error`	No	Error code when ok is false
`facts`	No
`notice`	No	Usage note, e.g. anonymous-tier limits
`target`	No	What was checked
`message`	No
`verdict`	No
`findings`	No
`headline`	No	One-line summary
`reportUrl`	No	Shareable report link

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 18, 2026

addedOutput schema / properties / notice

Added value: +{
+  "description": "Usage note, e.g. anonymous-tier limits",
+  "type": "string"
+}

Changed4 schema fields changedOct 18, 2026

addedInput schema / additionalProperties
```
Added value: +false
```

changedInput schema / properties / dkimSelector / description

Previous value: -"DKIM selector to check (optional)"New value: +"DKIM selector to check in addition to common ones, e.g. google, selector1, mail. Optional."

changedInput schema / properties / domain / description

Previous value: -"Mail domain, e.g. example.com"New value: +"Mail domain (the part after @), e.g. example.com"

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "properties": {
+    "error": {
+      "description": "Error code when ok is false",
+      "type": "string"
+    },
+    "facts": {
+      "items": {
+        "properties": {
+          "label": {
+            "type": "string"
+          },
+          "value": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "label",
+          "value"
+        ],
+        "type": "object"
+      },
+      "type": "array"
+    },
+    "findings": {
+      "items": {
+        "properties": {
+          "code": {
+            "type": "string"
+          },
+          "detail": {
+            "description": "Why it matters / cause",
+            "type": "string"
+          },
+          "docs": {
+            "description": "How-to article URL",
+            "type": "string"
+          },
+          "fix": {
+            "description": "How to fix",
+            "type": "string"
+          },
+          "severity": {
+            "enum": [
+              "error",
+              "warning",
+              "info",
+              "ok"
+            ],
+            "type": "string"
+          },
+          "title": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "severity",
+          "title"
+        ],
+        "type": "object"
+      },
+      "type": "array"
+    },
+    "headline": {
+      "description": "One-line summary",
+      "type": "string"
+    },
+    "message": {
+      "type": "string"
+    },
+    "ok": {
+      "description": "false when the check could not run (see error / message)",
+      "type": "boolean"
+    },
+    "reportUrl": {
+      "description": "Shareable report link",
+      "type": "string"
+    },
+    "target": {
+      "description": "What was checked",
+      "type": "string"
+    },
+    "tool": {
+      "type": "string"
+    },
+    "verdict": {
+      "enum": [
+        "ok",
+        "warn",
+        "fail"
+      ],
+      "type": "string"
+    }
+  },
+  "required": [
+    "ok"
+  ],
+  "type": "object"
+}

First observedOct 17, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint, and destructiveHint=false, so safety is covered. The description adds real substance beyond that: the breadth of checks performed, the 10-DNS-lookup SPF boundary, and the shape of the result (verdict, findings with cause and fix, report link). It stops short of disclosing rate limits or latency expectations for a live DNS/SMTP probe.

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?

Front-loaded with purpose and the scoping prefix 'as other mail servers see it,' with the record enumeration placed early. The long check list is dense but each item is a distinct capability rather than filler; only slightly heavy for a single audit tool.

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 an output schema present and annotations covering the safety profile, the description's remaining job is routing and scope, both of which are handled. Nothing an agent needs to select or invoke this tool correctly is missing.

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

Parameters3/5

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

Schema description coverage is 100%, so both params are already documented with examples (domain, dkimSelector). The description adds no syntax or constraint detail beyond that; the only adjacent hint is the SPF lookup limit, which is not a parameter. Baseline 3 is appropriate.

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 ('Audit') and resource ('a mail domain') framed by perspective ('as other mail servers see it'), then enumerates the exact record families inspected (MX, SPF, DKIM, DMARC, PTR/FCrDNS, STARTTLS, MTA-STS, TLS-RPT, BIMI). This clearly separates it from the generic sibling check_dns.

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

Usage Guidelines5/5

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

Gives concrete triggers (mail lands in spam or bounces, after changing mail provider or DNS, before tightening DMARC) and an explicit routing rule: use this, not check_dns, for any email deliverability or authentication question. Both when-to-use and the alternative are named.

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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