Sendmux Email Inbox API + Sending

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Send Mailbox Message

qubrics_mailbox_send_message

Send an email from the authenticated mailbox with recipients, subject, HTML or plain text, and attachments; upload attachment blobs first and include an Idempotency-Key for safe retries.

Instructions

Use this to send a message from the authenticated mailbox. For attachments, call mailbox_upload_attachment first using presign_upload_url or tiny content_base64, then pass the returned blob_id. Include an Idempotency-Key for retries.

Input Schema

Name	Required	Description
`cc`	No	Visible copied recipients.
`to`	No	Primary recipients. Supply 1 to 50 recipients in total across To, Cc and Bcc.
`bcc`	No	Hidden copied recipients.
`from`	No	Sender address. Defaults to mailbox identity.
`subject`	No	Subject line; defaults to empty.
`reply_to`	No	Reply-To recipients.
`html_body`	No	HTML body.
`text_body`	No	Plain text body.
`mailbox_id`	No	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.
`attachments`	No	Attachments to send with the message.
`custom_headers`	No	Custom X-* headers to include. Mailbox writes reject X-SES-TENANT, X-SES-CONFIGURATION-SET and X-SES-MESSAGE-TAGS case-insensitively.
`Idempotency-Key`	No	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.

Output Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Addedv1.0.17Nov 7, 2026

TDQS

A3.7/5.0

Behavior3/5

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

The annotations already establish that this is a non-read-only, open-world, non-idempotent operation. The description adds attachment pre-upload sequencing and retry-key behavior, but it does not mention that sending is effectively irreversible, nor does it cover permission or rate-limit considerations.

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?

The description is three short sentences with no wasted wording. The main action is front-loaded, followed by the attachment workflow and retry guidance.

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?

Given 12 parameters, full schema coverage, and an output schema, the description covers the important non-obvious workflows: attachments and retries. It could still say more about use versus draft-based sending and delivery irreversibility.

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

Parameters4/5

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

Schema coverage is 100%, so the baseline is 3. The description still adds value by explaining the attachment workflow and idempotency-key usage beyond the schema's individual property descriptions.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource: sending a message from the authenticated mailbox. It is clear, but it does not differentiate this tool from close siblings such as mailbox_send_draft or sending_send_email.

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?

It gives useful attachment guidance: upload first, then pass the returned blob_id. It also mentions Idempotency-Key for retries, but it does not explain when to use this tool versus creating or sending a draft, or versus the sending_* variants.

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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