mcp-toolbelt

smtp_server_check

Live-probe a mail server over SMTP: connects to port 25 (or a given port), reads the banner, issues EHLO and lists advertised capabilities (STARTTLS, AUTH mechanisms, SIZE, 8BITMIME, PIPELINING), then upgrades via STARTTLS when offered and reports the TLS protocol and certificate (issuer, expiry). The server-side complement to email_deliverability_check (which reads DNS records only and never connects). No mail is sent and no relay attempt is made.

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": {
      "host": "<host>"
    },
    "name": "smtp_server_check"
  }
}

Input Schema

Name	Required	Description	Default
`host`	Yes	Mail server hostname, e.g. mail.example.com — or a bare domain, in which case its lowest-priority-number MX host is probed
`port`	No	SMTP port (default 25)

Schema Changelog

Changes observed during successful MCP inspections.

AddedOct 21, 2026

TDQS

A4.4/5.0

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 thoroughly: it discloses the connection target and default port, the exact protocol handshake sequence, the STARTTLS upgrade behavior, what is reported back, and the crucial side-effect guarantee that 'no mail is sent and no relay attempt is made.' Nothing about the tool's behavior is left to inference.

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 core purpose is front-loaded in the first clause and the closing sentence delivers the highest-value fact (no mail sent, no relay). The long middle sentence packs the handshake sequence densely but every clause earns its place; only mild trimming is possible.

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?

No output schema and no annotations exist, yet the description still tells the agent what comes back (banner, advertised capabilities such as STARTTLS/AUTH/SIZE/8BITMIME/PIPELINING, TLS protocol and certificate issuer/expiry). For a 2-param probe tool, that is complete.

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 parameters (host with MX fallback semantics, port with default 25) are already fully documented in the schema. The description only restates the port default ('port 25 (or a given port)') and adds no syntax or format detail beyond it. Baseline 3 applies when the schema does the heavy lifting.

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?

Opens with a specific verb+resource ('Live-probe a mail server over SMTP') and then enumerates exactly what the probe does: banner read, EHLO, capability listing, STARTTLS upgrade, TLS/cert report. It is unmistakably distinct from siblings like check_open_ports or email_deliverability_check.

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?

Explicitly positions itself as the server-side complement to email_deliverability_check, stating that the sibling 'reads DNS records only and never connects' — a clear routing rule. It stops short of stating exclusion cases (e.g., non-SMTP services should use check_open_ports instead), so it is strong but not exhaustive.

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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