Sendmux

What is Sendmux?

Sendmux is an email infrastructure service built for AI agents, giving each agent a dedicated inbox it can read from and send from through one API. Incoming messages arrive as structured JSON with signatures and quoted history removed by default, so an agent spends fewer tokens parsing mail. On the sending side, Sendmux works as a routing layer that spreads traffic across connected providers such as Gmail, SendGrid, Mailgun, Resend or any SMTP service, with quotas and automatic failover when one provider fails.

The service is run by Jonah and Associates Pty Ltd, a company based in Melbourne, Australia, according to the official site. AI Agents Directory listed Sendmux on 26 June 2026, and in July 2026 the team released a free MailWizz extension that adds Sendmux as a delivery server type. Developers connect through HTTP and SMTP APIs, SDKs, a CLI or an MCP endpoint for Claude Code. Sendmux was live with a free tier and a usage-based Pro plan in October 2026.

Key Features

• Dedicated agent mailboxes: Sendmux gives each agent its own inbox on the @myagent.mx domain or on a custom domain you connect. A single dashboard lists every agent inbox, so a team can watch what each agent receives and sends.
• Clean JSON messages: Sendmux converts raw HTML email into structured JSON and strips signatures and quoted history by default. The homepage example shows a 3,247-token raw email reduced to 127 tokens, a vendor figure of about 96% saved.
• Multi-provider sending: Sendmux sends through the HTTP API or SMTP on port 587 and can route through Gmail, Outlook, SendGrid, Mailgun, Resend or generic SMTP. Directory listings also mention a managed Amazon SES option for teams without their own provider.
• Weighted routing and failover: Sendmux distributes outbound mail across providers by weight and reroutes traffic when one provider goes down. Delivery groups can also route by sender domain, which helps teams that send from several brands.
• Four-level quota controls: Sendmux lets you cap sending per second, minute, hour and day for each route or account. Health monitoring disables a failing provider automatically and re-enables it once it recovers.
• Real-time events: Sendmux streams inbox activity through Server-Sent Events and posts delivery events to signed webhooks. Agents can react to a new message as it lands instead of polling the inbox on a timer.
• Batch message actions: Sendmux applies actions to up to 100 message IDs in one request. The message content API can also return optional link, header and attachment metadata when an agent needs more than the body text.
• Developer tooling and MCP: Sendmux offers SDKs for TypeScript, Python, Go, PHP and Ruby, according to directory listings, plus a CLI. Its MCP server at mcp.sendmux.ai lets Claude Code and other MCP clients use mailboxes directly.
• Team isolation and scoped keys: Sendmux supports multi-tenancy with separate teams and API keys limited to a single inbox. A key leaked from one agent therefore cannot read or send from the other mailboxes in the account.

Pros

• Generous free start: Sendmux offers a $0 plan with two mailboxes and 50 accepted recipients a day, with no card required. Developers can prototype an email-capable agent before committing to any spend.
• Low, transparent usage rates: Sendmux charges Pro users $7 per team monthly plus per-event fees from $0.0005, with no per-seat or per-mailbox charges. Teams running many agent inboxes avoid paying a flat fee for every address.
• Fewer tokens per email: Sendmux trims signatures, quoted replies and HTML before an agent sees a message. Smaller payloads lower model costs and reduce the chance that an agent misreads a long, nested thread.
• Bring your own providers: Sendmux routes mail through accounts you already own, such as Gmail, Outlook, SendGrid or Mailgun. You keep existing sender reputations and can spread volume across several providers with failover.
• Framework-friendly: Sendmux documents integrations for Claude Code, LangChain, the Vercel AI SDK, Node.js and Python. Agent builders can plug email into an existing stack rather than writing IMAP and SMTP handling themselves.

Cons

• Very new service: Sendmux appeared in AI directories in mid-2026, and its Trustpilot page showed no reviews when we checked. Long-term reliability, support quality and pricing stability have little public track record yet.
• Developer-only product: Sendmux has no visual campaign builder or templates for marketers. You need to work with APIs, SDKs, webhooks or MCP, so non-technical teams will need a developer to set it up.
• Tight free sending cap: Sendmux limits the free plan to 50 accepted recipients a day across all routes. Any agent that handles real customer volume will move onto usage billing quickly.
• Usage costs need monitoring: Sendmux bills Pro usage per event and per gigabyte of mailbox storage, on top of the team fee. Busy inboxes and high send volumes can make monthly bills harder to predict than a flat plan.
• Deliverability rests on you: Sendmux routes mail through your connected providers, so sender reputation, domain authentication and provider rules remain your responsibility. Spreading cold outreach across many accounts can still breach those providers’ terms.

Who is Using Sendmux?

• AI agent developers: Give agents inboxes to handle sign-ups, replies and email tasks.
• SaaS engineering teams: Route transactional mail across providers with failover.
• MailWizz operators: Spread campaigns across Gmail, Outlook and SMTP accounts.
• Automation builders: Trigger agent workflows from incoming email events.

Pricing

Sendmux uses a freemium model with a free plan and a Pro plan billed per team plus usage (official homepage prices, October 2026).

Sendmux charges no per-seat or per-mailbox fees, and directories describe the model as freemium with usage-based billing. Usage rates on new infrastructure products can change quickly, so check the live pricing page and estimate your monthly event volume first.

What Makes Sendmux Unique?

Sendmux combines two jobs that agent builders usually solve with separate tools: a receiving inbox that hands agents token-light JSON, and a sending router that balances mail across your own providers. Most transactional email APIs focus on outbound delivery from one provider, while Sendmux treats Gmail, Outlook, SendGrid, Mailgun and SMTP accounts as a pool with weights, quotas and failover. Its MCP endpoint and inbox-scoped keys show the product is designed around autonomous agents rather than human marketers.

Compatibilities and Integrations

• Platforms: Web dashboard plus HTTP and SMTP APIs; a CLI is available for terminal use.
• Providers: Gmail, Outlook, SendGrid, Mailgun, Resend, generic SMTP and, per directories, managed Amazon SES.
• Agent frameworks: Claude Code through MCP, LangChain, the Vercel AI SDK, Node.js and Python, with further SDKs listed for Go, PHP and Ruby.
• Events and plugins: Signed webhooks, Server-Sent Events and a free MailWizz extension on GitHub for MailWizz 2.0.0 and PHP 8.0 or later.

Sendmux Tutorials

A first agent inbox takes a few steps from sign-up to live mail.

1. Create a free Sendmux account at sendmux.ai; no card is needed.
2. Create a mailbox on @myagent.mx or connect your own domain.
3. Generate an inbox-scoped API key and install the SDK for your language, or add the MCP endpoint to Claude Code.
4. Connect a sending provider such as Gmail, SendGrid or SMTP and set per-minute and per-day quotas.
5. Subscribe to webhooks or SSE so your agent reacts to new messages, then send a test email.

How We Rated It

Summary

Sendmux suits developers who want their AI agents to read and send real email without building IMAP parsing or provider failover themselves. Its main limitation is maturity: the service is new, reviews are scarce and everything runs through code. Pricing is low for light use, but busy inboxes add per-event charges. Start on the free plan with one test agent, measure your daily event count, and only then connect production sending accounts.