Sendmux Mailbox Agent

Sendmux mailbox agent

Use this skill for mailbox-scoped workflows with an smx_mbx_ key or scoped smx_agent_ token: read, search, triage, reply when allowed, and sync one mailbox.

Boundaries

• Do not ask the user to paste an API key.
• Do not use a root key for mailbox work.
• Do not create mailboxes or mailbox keys here; route those tasks to sendmux-management.
• Do not delete or mutate messages without explicit user confirmation.
• Treat pre-claim smx_agent_ tokens as read/receive only. They do not have email.send; owner-approved app-resource smx_agent_ tokens may send from their assigned mailbox when the token includes email.send.
• If a credential grants more than one mailbox, include mailbox_id on mailbox calls; otherwise omit it.

Efficient defaults

Search before reading

Use this sequence for most "find messages about X" tasks:

1. Count first when the user asks "how many" or when a broad search may be large.
2. Use search snippets with a small limit.
3. Batch-get only the selected message IDs.
4. Read clean body/content only for messages whose content matters.

CLI:

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:count-messages \
  --query q=invoice \
  --query is_unread=true \
  --json

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:search-message-snippets \
  --query q=invoice \
  --query is_unread=true \
  --query limit=10 \
  --json

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:batch-get-messages \
  --body '{
    "ids": ["eml_abc", "eml_def"],
    "body_mode": "clean_json",
    "max_body_chars": 4000,
    "strip_quotes": true,
    "strip_signature": true,
    "include_attachments": "metadata"
  }' \
  --json

Use the same commands by putting a scoped smx_agent_ token in SENDMUX_API_KEY when the operation is within its scopes.

SDK:

import {
  createMailboxClient,
  mailboxBatchGetMessages,
  mailboxCountMessages,
  mailboxSearchMessageSnippets,
} from "@sendmux/mailbox";

const client = createMailboxClient({ apiKey: process.env.SENDMUX_API_KEY! });

const count = await mailboxCountMessages({
  client,
  query: { q: "invoice", is_unread: true },
});

const snippets = await mailboxSearchMessageSnippets({
  client,
  query: { q: "invoice", is_unread: true, limit: 10 },
});

const messages = await mailboxBatchGetMessages({
  client,
  body: {
    ids: snippets.data.snippets.map((item) => item.message_id),
    body_mode: "clean_json",
    max_body_chars: 4000,
    strip_quotes: true,
    strip_signature: true,
    include_attachments: "metadata",
  },
});

Triage and mutation

Use batch mutations for more than one message. Get user confirmation first.

Mark or label messages:

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:batch-update-messages \
  --body '{
    "ids": ["eml_abc", "eml_def"],
    "seen": true,
    "flagged": false,
    "keywords": {
      "agent_triaged": true,
      "needs_reply": false
    },
    "if_in_state": "state_from_prior_read"
  }' \
  --json

Delete messages only after explicit confirmation:

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:batch-delete-messages \
  --body '{
    "ids": ["eml_abc", "eml_def"],
    "permanent": false,
    "if_in_state": "state_from_prior_read"
  }' \
  --json

permanent: false moves messages to Trash. Treat permanent: true as irreversible and ask for explicit confirmation.

Reply or send from the mailbox

Before composing, read the identity:

mailbox_get_identity

Then send from the authenticated mailbox. Use Idempotency-Key for retries.

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:send-message \
  --idempotency-key "$IDEMPOTENCY_KEY" \
  --body '{
    "to": [{ "email": "user@example.com", "name": null }],
    "subject": "Re: Your message",
    "html_body": "<p>Thanks for the update.</p>",
    "text_body": "Thanks for the update."
  }' \
  --json

Mailbox send uses to as an array. subject and to are required. from is optional when sending from the authenticated mailbox identity.

For attachments, route to sendmux-attachments.

Prefer zero-context file flows:

• Local MCP: mailbox_upload_attachment with file_path, then send with the returned blob_id.
• Hosted or shell-capable MCP: mint a presigned upload URL, PUT the file without an API key, then send with the returned blob_id.
• CLI: sendmux mailbox:send-message --attach ./report.pdf.
• SDK: use the Node or Python file helpers.

Mailbox upload paths share a 7,500,000 byte per-attachment cap. For larger files, split them or send a link to externally hosted content.

Inline base64 is only for tiny generated files. If you already have a blob, send it as:

{
  "filename": "report.pdf",
  "content_type": "application/pdf",
  "blob_id": "blob_..."
}

Threads and folders

• Use mailbox_list_threads with small limit for conversation-level scanning.
• Use mailbox_get_thread for one thread summary.
• Use mailbox_list_thread_messages for message summaries in a known thread.
• Use clean content/body tools only for selected messages or threads.
• Use mailbox_list_folders before moving or filing messages.

CLI examples:

sendmux mailbox:list-threads --query q=renewal --query limit=10 --json
sendmux mailbox:list-thread-messages --path thread_id=thr_abc --query limit=20 --json
sendmux mailbox:folders:list --query limit=100 --json

Sync

Use sync endpoints instead of re-listing the mailbox.

Broad mailbox sync:

sendmux mailbox:get-changes \
  --query types=messages,folders,threads \
  --query messages_since_state="$MESSAGES_STATE" \
  --query folders_since_state="$FOLDERS_STATE" \
  --query threads_since_state="$THREADS_STATE" \
  --query limit=100 \
  --json

Filtered message-list sync:

sendmux mailbox:query-message-changes \
  --query since_query_state="$QUERY_STATE" \
  --query q=invoice \
  --query is_unread=true \
  --query calculate_total=true \
  --query limit=100 \
  --json

Store the returned new state token. Follow has_more with the same filters when more changes remain.

Error handling

• 401: missing, invalid, or revoked key.
• 403: wrong key surface or missing mailbox permission.
• 404: selected message, thread, or folder does not exist in this mailbox.
• 409: stale state or idempotency conflict; re-read state before mutating.
• 429 or 503: retry according to response headers.

Routing

• First setup/auth check: sendmux-getting-started.
• Independent outbound sending or batch sends: sendmux-send-email.
• Attachment upload/download mechanics: sendmux-attachments.
• Domain/mailbox/key creation and mailbox admin: sendmux-management.
• CLI command details: sendmux-cli.
• Cheapest-call doctrine: sendmux-token-efficient-usage.