langchain-sendmux

LangChain toolkit for Sendmux, the email API for AI agents.

Gives an agent its own mailbox: it can send email, read what arrives, and reply from its own address.

Requirements

Python 3.10 or newer
langchain-core v1

Installation

pip install langchain-sendmux

Getting an API key

The toolkit needs a key that can both send and receive. Two ways to get one:

Dashboard — create a mailbox and a mailbox-scoped key (smx_mbx_*). See API keys.
Agent self-registration — the agent claims its own @myagent.mx mailbox and gets an smx_agent_* token, with no human signup first. See email for AI agents.

Note on agent tokens: a freshly self-registered smx_agent_* token can read and receive, but cannot send until a human owner has been invited and has approved it. Until then send_email and reply will fail. A dashboard smx_mbx_* key with send permission works immediately.

Read the key from the environment. Never hard-code it.

Quick start

import os

from langchain.agents import create_agent

from langchain_sendmux import SendmuxToolkit

smx = SendmuxToolkit(
    api_key=os.environ["SENDMUX_API_KEY"],
    default_from="agent@yourdomain.dev",
)

agent = create_agent(
    model="gpt-4o",
    tools=smx.get_tools(),
    system_prompt="You triage the support inbox.",
)

agent.invoke(
    {"messages": [{"role": "user", "content": "Reply to anyone asking about pricing."}]}
)

Tools

SendmuxToolkit(...).get_tools() returns three tools.

`send_email`

Sends through your configured sending providers, to any recipient.

Parameter	Type	Required	Notes
`to`	str	yes	Recipient email address
`subject`	str	yes	Subject line
`text`	str	yes	Plain-text body
`html`	str	no	HTML body. Generated from `text` if omitted
`var_from`	str	no	Sender address. Falls back to `default_from`
`idempotency_key`	str	no	Makes a retried send idempotent for 24 hours

var_from is spelt that way because from is a reserved word in Python.

`list_messages`

Lists messages in the agent's own mailbox, newest first.

Parameter	Type	Required	Notes
`limit`	int	no	How many to return, 1 to 100. Defaults to 25

`reply`

Sends from the agent's own mailbox address, rather than through a sending provider. Use this to answer someone who wrote in.

Parameter	Type	Required	Notes
`to`	str	yes	Recipient email address
`subject`	str	yes	Subject line
`text`	str	yes	Plain-text body
`html`	str	no	HTML body. Generated from `text` if omitted
`idempotency_key`	str	no	Makes a retried send idempotent for 24 hours

Configuration

SendmuxToolkit(api_key=..., default_from=...)

Option	Required	Purpose
`api_key`	yes	A send + receive mailbox key (`smx_mbx_`) or a scoped agent token (`smx_agent_`)
`default_from`	no	Default sender for `send_email`. Without it, the model has to supply `var_from` on every call

Retries and duplicate sends

Agents retry. Pass idempotency_key on send_email and reply and Sendmux will send once, even if the same call arrives several times inside 24 hours. Any stable string works — a task id, a thread id, a hash of the message.

Details: idempotency.

Common errors

What you see	Why	Fix
`ValueError: No sender address`	No `var_from` on the call and no `default_from` set	Set `default_from`, or have the model pass `var_from`
Send rejected on an agent token	The token is self-registered and not yet owner-approved	Complete the owner invite and approval
Auth failure	Key lacks send or receive permission	Check the key's scope in the dashboard

Quickstart
Mailboxes
All Sendmux SDKs
Pricing — usage-based, no per-seat or per-mailbox fees

Licence

MIT. See the licence file.

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This release

0.2.1

Aug 7, 2026

0.2.0

Jul 22, 2026

langchain-sendmux 0.2.1

langchain-sendmux

Requirements

Installation

Getting an API key

Quick start

Tools

`send_email`

`list_messages`

`reply`

Configuration

Retries and duplicate sends

Common errors

Related

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1 maintainer

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