Most email integrations only go outward, which means an agent can ask a question and never learn the answer. mailbox_wait_for_message closes that loop — send, wait, read the reply, act on it — and that is the difference between notification and correspondence. The three surfaces are worth taking seriously as a boundary: an agent that only needs to send should hold a Sending grant and nothing else, because a root key reaches domains, billing and every mailbox on the account.
Sendmux's SDK repository, which includes the MCP server for its email platform. What is unusual is the shape: an agent gets a real mailbox it can search, read and reply from, not just a send endpoint. The tools are split into three surfaces and you grant them separately.
- Mailbox — mailbox_list_messages, mailbox_list_threads, mailbox_get_message, mailbox_get_thread and mailbox_search_message_snippets for finding things; mailbox_list_folders and mailbox_batch_update_messages for organising them
- mailbox_wait_for_message and mailbox_get_changes, which is what lets an agent send something and then wait for the reply rather than polling blindly
- Attachments both directions — mailbox_upload_attachment, mailbox_get_attachment and mailbox_read_attachment
- Sending — sending_send_email and sending_send_email_batch, with sending_create_attachment_upload and sending_upload_attachment behind them
- Management — management_create_domain, management_verify_domain and management_get_domain_zone_file for setting up a sending domain; management_create_mailbox, management_update_mailbox, management_suspend_mailbox and management_create_mailbox_key for the mailboxes themselves
- Deliverability and cost — management_list_email_logs, management_get_email_log, management_get_email_metrics and management_get_spend_summary
- Webhooks — management_create_webhook, management_list_webhooks and management_test_webhook
- A separate documentation server exposing search_sendmux, for searching Sendmux's own guides and API reference
- Toolkits for other frameworks from the same repository, including a LangChain package and Vercel AI SDK tools
A Sendmux account. The hosted server at https://mcp.sendmux.ai/mcp authorises through https://app.sendmux.ai and you choose which surfaces the grant covers. Running it locally means pip install sendmux-mcp, naming the surfaces in SENDMUX_MCP_SURFACES, and supplying keys scoped to what you are enabling — a mailbox key for Mailbox, a root key for Management, and for Sending either a mailbox key or an owner-approved agent token. Over HTTP transport SENDMUX_MCP_HTTP_BEARER_TOKEN is required. Sending real email means a verified domain and Sendmux's own pricing.
One command plus a key — pip install sendmux-mcp, then supply credentials