> ## Documentation Index
> Fetch the complete documentation index at: https://sendmux.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Send a mailbox message

> Creates and queues a message from the authenticated mailbox. Supply an `Idempotency-Key` header to safely retry. Attachments may use small inline base64 content or blob IDs returned by `POST /mailbox/attachments:upload` or `POST /mailbox/attachment-uploads`.


## OpenAPI

````yaml /openapi-app.json post /mailbox/messages/send
openapi: 3.1.0
info:
  description: Programmatic access to your Sendmux email infrastructure.
  title: Sendmux API
  version: 1.0.0
servers:
  - description: API v1
    url: https://app.sendmux.ai/api/v1
security:
  - bearerAuth: []
paths:
  /mailbox/messages/send:
    post:
      tags:
        - Mailbox API
      summary: Send a mailbox message
      description: >-
        Creates and queues a message from the authenticated mailbox. Supply an
        `Idempotency-Key` header to safely retry. Attachments may use small
        inline base64 content or blob IDs returned by `POST
        /mailbox/attachments:upload` or `POST /mailbox/attachment-uploads`.
      operationId: mailboxSendMessage
      parameters:
        - in: header
          name: Idempotency-Key
          required: false
          schema:
            description: >-
              Client-chosen unique key to safely retry the request. Cached for
              24h per (mailbox, endpoint, key). Different body with same key
              returns 409 idempotency_conflict.
            maxLength: 255
            type: string
        - description: >-
            Mailbox public ID to target when the credential grants access to
            more than one mailbox. Omit when the credential is scoped to exactly
            one mailbox.
          in: query
          name: mailbox_id
          required: false
          schema:
            type: string
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/SendMailboxMessageBody'
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/MailboxSendResultResponse'
          description: Message queued
        '400':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ApiError'
          description: Invalid request body
        '409':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ApiError'
          description: Idempotency conflict
        '413':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ApiError'
          description: Request body too large
        '422':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ApiError'
          description: Message validation failed
        '503':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ApiError'
          description: Mailbox service unavailable
      security:
        - bearerAuth: []
components:
  schemas:
    SendMailboxMessageBody:
      additionalProperties: false
      properties:
        attachments:
          description: Attachments to send with the message.
          items:
            anyOf:
              - additionalProperties: false
                properties:
                  content:
                    description: >-
                      Base64-encoded content for small inline attachments. Use
                      /mailbox/attachments:upload for larger files.
                    type: string
                  content_type:
                    type: string
                  filename:
                    type: string
                required:
                  - filename
                  - content_type
                  - content
                type: object
              - additionalProperties: false
                properties:
                  blob_id:
                    description: Blob ID returned by /mailbox/attachments:upload.
                    type: string
                  content_type:
                    type: string
                  filename:
                    type: string
                required:
                  - filename
                  - content_type
                  - blob_id
                type: object
          type: array
        bcc:
          description: BCC recipients.
          items:
            $ref: '#/components/schemas/MailboxAddress'
          type: array
        cc:
          description: CC recipients.
          items:
            $ref: '#/components/schemas/MailboxAddress'
          type: array
        custom_headers:
          additionalProperties:
            type: string
          description: Custom headers to include.
          type: object
        from:
          allOf:
            - $ref: '#/components/schemas/MailboxAddress'
            - description: Sender address. Defaults to mailbox identity.
        html_body:
          description: HTML body.
          type: string
        reply_to:
          description: Reply-To recipients.
          items:
            $ref: '#/components/schemas/MailboxAddress'
          type: array
        subject:
          description: Subject line for the outgoing email.
          type: string
        text_body:
          description: Plain text body.
          type: string
        to:
          description: Primary recipients.
          items:
            $ref: '#/components/schemas/MailboxAddress'
          minItems: 1
          type: array
      required:
        - to
        - subject
      type: object
    MailboxSendResultResponse:
      allOf:
        - $ref: '#/components/schemas/SuccessEnvelope'
        - properties:
            data:
              $ref: '#/components/schemas/SendMailboxMessageResult'
            meta:
              $ref: '#/components/schemas/ResponseMeta'
          required:
            - data
          type: object
      unevaluatedProperties: false
    ApiError:
      additionalProperties: false
      properties:
        error:
          additionalProperties: false
          properties:
            code:
              description: Machine-readable error code
              enum:
                - invalid_parameter
                - missing_parameter
                - authentication_required
                - insufficient_permissions
                - not_found
                - conflict
                - limit_exceeded
                - idempotency_conflict
                - payload_too_large
                - validation_error
                - rate_limit_exceeded
                - service_unavailable
                - internal_error
              example: invalid_parameter
              type: string
            doc_url:
              description: Link to relevant documentation
              type: string
            errors:
              description: >-
                Accumulated per-field issues for validation errors. Only present
                on 400/422 responses.
              items:
                $ref: '#/components/schemas/ApiErrorDetail'
              type: array
            message:
              description: Human-readable error description
              type: string
            param:
              description: The parameter that caused the error
              type: string
            retryable:
              description: >-
                Whether the caller may safely retry this request. 4xx are
                typically false (except 429); 5xx are typically true.
              example: false
              type: boolean
          required:
            - code
            - message
            - retryable
          type: object
        meta:
          additionalProperties: false
          properties:
            request_id:
              example: req_clxxxxxxxxxxxxxxxxxxxxxxxxx
              type: string
          required:
            - request_id
          type: object
        ok:
          enum:
            - false
          type: boolean
      required:
        - ok
        - error
        - meta
      type: object
    MailboxAddress:
      additionalProperties: false
      properties:
        email:
          example: agent@example.com
          format: email
          type: string
        name:
          example: Support
          type:
            - string
            - 'null'
      required:
        - email
        - name
      type: object
    SuccessEnvelope:
      properties:
        meta:
          additionalProperties: {}
          properties: {}
          type: object
        ok:
          enum:
            - true
          type: boolean
      required:
        - ok
        - meta
      type: object
    SendMailboxMessageResult:
      additionalProperties: false
      properties:
        message_id:
          type: string
        status:
          enum:
            - queued
          type: string
      required:
        - message_id
        - status
      type: object
    ResponseMeta:
      additionalProperties: false
      properties:
        request_id:
          example: req_clxxxxxxxxxxxxxxxxxxxxxxxxx
          type: string
      required:
        - request_id
      type: object
    ApiErrorDetail:
      additionalProperties: false
      properties:
        code:
          description: >-
            Machine-readable issue code from the validator (e.g. zod issue
            code).
          example: invalid_string
          type: string
        field:
          description: Dot-path of the offending request field.
          example: recipient.email
          type: string
        message:
          description: Human-readable issue description.
          type: string
      required:
        - field
        - code
        - message
      type: object
  securitySchemes:
    bearerAuth:
      bearerFormat: API Key
      description: >-
        Sendmux API key. Use a root API key for Management API routes, or a
        mailbox credential for Mailbox API routes. Obtain keys from the
        dashboard under API Keys.
      scheme: bearer
      type: http

````