Sendmux Email Inbox API + Sending

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by Sendmux

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Hybrid

List Granted Mailboxes

mailbox_list_granted_mailboxes

Read-onlyIdempotent

Retrieve mailboxes accessible to this connection and return their IDs to use as mailbox_id in API calls.

Instructions

Use this first when the token can access more than one mailbox. It searches only the mailboxes granted to this connection and returns IDs to pass as mailbox_id.

Input Schema

TableJSON Schema

Name	Required	Description
`q`	No	Search by mailbox ID or email address.
`limit`	No	Maximum results (default 50).
`cursor`	No	Cursor from the previous response.

Output Schema

TableJSON Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed4 schema fields changedv1.0.8Sep 15, 2026

addedOutput schema / $schema

Added value: +"https://json-schema.org/draft/2020-12/schema"

removedOutput schema / additionalProperties
```
Removed value: -true
```

addedOutput schema / properties

Added value: +{
+  "result": {
+    "allOf": [
+      {
+        "properties": {
+          "meta": {
+            "additionalProperties": {},
+            "properties": {},
+            "type": "object"
+          },
+          "ok": {
+            "enum": [
+              true
+            ],
+            "type": "boolean"
+          }
+        },
+        "required": [
+          "ok",
+          "meta"
+        ],
+        "type": "object"
+      },
+      {
+        "properties": {
+          "data": {
+            "items": {
+              "additionalProperties": false,
+              "properties": {
+                "email": {
+                  "description": "Mailbox email address",
+                  "example": "agent@acme.com",
+                  "type": "string"
+                },
+                "id": {
+                  "description": "Mailbox public ID",
+                  "example": "mbx_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+                  "type": "string"
+                }
+              },
+              "required": [
+                "id",
+                "email"
+              ],
+              "type": "object"
+            },
+            "type": "array"
+          },
+          "meta": {
+            "additionalProperties": false,
+            "properties": {
+              "request_id": {
+                "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+                "type": "string"
+              }
+            },
+            "required": [
+              "request_id"
+            ],
+            "type": "object"
+          },
+          "pagination": {
+            "additionalProperties": false,
+            "properties": {
+              "has_more": {
+                "type": "boolean"
+              },
+              "next_cursor": {
+                "type": "string"
+              }
+            },
+            "required": [
+              "has_more"
+            ],
+            "type": "object"
+          }
+        },
+        "required": [
+          "data",
+          "pagination"
+        ],
+        "type": "object"
+      }
+    ],
+    "unevaluatedProperties": {
+      "not": {}
+    },
+    "x-fastmcp-top-level-schema": "GrantedMailboxListResponse"
+  }
+}

addedOutput schema / required
```
Added value: +[
+  "result"
+]
```

First observedv1.0.0Jul 13, 2026

TDQS

A4.3/5.0

Behavior4/5

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

Annotations already cover read-only, idempotent, and non-destructive behavior. The description adds useful behavioral context: it searches only the granted mailbox set and returns IDs intended for mailbox_id, which goes beyond the annotations without contradicting them.

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?

Two short sentences front-load the usage condition ('Use this first') before stating scope and output. Every sentence earns its place with no redundancy or 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?

For a simple read-only list/search tool with fully documented optional parameters and an output schema, the description covers why to use it, when to use it, its scope, and how its result is consumed downstream. Nothing essential is missing for correct invocation.

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?

The input schema provides full descriptions for q, limit, and cursor, with 100% coverage. The description adds no parameter-level semantics beyond that, so the schema carries the load and the baseline of 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?

The description states a specific verb ('searches') and resource ('mailboxes granted to this connection'), and clarifies the output: IDs to pass as mailbox_id. 'Use this first' positions it as the discovery tool among sibling mailbox operations, making its purpose distinct.

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 explicitly says to use this tool first when the token can access more than one mailbox, giving a clear trigger condition. It does not name alternatives or state when not to use it, so it stops short of full exclusion guidance.

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