domain-security-mcp-server

MTA-STS Check

mta_sts_check

Read-onlyIdempotent

Audit a domain's MTA-STS deployment by checking its _mta-sts TXT record and policy file, then verify enforcement mode (enforce/testing/none) and MX hosts to detect TLS downgrade risks.

Instructions

Check a domain's MTA-STS deployment: the _mta-sts TXT record AND the policy file at https://mta-sts./.well-known/mta-sts.txt. Reports the enforcement mode (enforce/testing/none) and the listed MX hosts. MTA-STS forces TLS for inbound SMTP and blocks downgrade attacks.

Args:

domain (string): the domain to check.
response_format ('markdown' | 'json'): output format (default 'markdown').

Returns: { dns_record_found, policy_found, mode, policy{}, findings[] }.

Example: "Does gmail.com enforce MTA-STS?" -> mta_sts_check(domain="gmail.com").

Input Schema

TableJSON Schema

Name	Required	Description	Default
`domain`	Yes	Domain to check, e.g. 'example.com'.
`response_format`	No	Output format: 'markdown' for a human-readable summary (default) or 'json' for the full structured payload.	markdown

Output Schema

TableJSON Schema

Name	Required	Description	Default
`mode`	No
`domain`	Yes
`policy`	No
`findings`	Yes
`policy_found`	Yes
`dns_record_found`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed5 schema fields changedv1.2.1Sep 14, 2026

changedInput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

removedInput schema / additionalProperties
```
Removed value: -false
```
addedInput schema / properties / domain / maxLength
```
Added value: +253
```

changedOutput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

addedOutput schema / properties / policy / propertyNames
```
Added value: +{
+  "type": "string"
+}
```

First observedv1.0.0Jun 8, 2026

TDQS

A3.9/5.0

Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds behavioral context by explaining what MTA-STS does (forces TLS, blocks downgrade attacks) and what the check entails (TXT record + policy fetch). It does not contradict annotations.

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?

The description is well-structured: a concise opening sentence, an Args section, and a Returns line. It is front-loaded with the core purpose and includes a helpful example. Slightly wordy in the middle but overall efficient.

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

Completeness4/5

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

The tool has an output schema and annotations cover safety. The description still provides a brief return shape ({ dns_record_found, policy_found, mode, policy{}, findings[] }) and an example invocation, which is sufficient for an agent to call it correctly. Minor gap: no mention of timeouts or DNS propagation, but not essential given the annotations.

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% – both domain and response_format have clear descriptions, including enum values and defaults. The description's Args section essentially repeats schema info without adding new semantics. Baseline of 3 applies since the schema carries the parameter documentation.

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?

The description clearly states the tool checks a domain's MTA-STS deployment, specifying both the TXT record and policy file, and what it reports (enforcement mode, MX hosts). It is a specific verb+resource and distinct from siblings like spf_check or dmarc_check, which target different protocols.

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

Usage Guidelines3/5

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

The description explains what the tool does and provides an example, but it does not explicitly contrast it with alternatives like email_auth_audit or mention when NOT to use it. Usage context is implied (when you need MTA-STS status) but no exclusions or sibling differentiation are stated.

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