SendMux

What is SendMux?

SendMux is an email API built specifically for AI agents. Instead of stitching together a transactional sender, an IMAP inbox, a webhook parser, and a pile of glue code, you get one control plane that lets an agent own a real inbox, send through your existing providers, and react to incoming mail in real time. It's the plumbing that turns "my agent can call an API" into "my agent has an email address people can actually reply to."

Here's the problem it solves. Most email APIs were designed for humans pushing marketing blasts or a SaaS app firing password resets. Agents need something different: a mailbox they can poll, threads they can read without wading through HTML signatures, and a way to send a reply through Gmail or Amazon SES without you rebuilding delivery logic every time. SendMux was built for that from day one, and it shows in the details — clean JSON responses, quoted history stripped by default, and a Model Context Protocol server so an agent can call it directly.

Who is it for? Two clear audiences. First, AI agent builders who need their agents to send and receive email under a verified domain. Second, SaaS and platform teams who want to give every customer an isolated team with its own scoped keys and mailboxes. If you're just sending the occasional transactional email from a web app, a plain sender is fine. If email is a first-class channel your agents live in, SendMux is a different category of tool.

Key Features

We spent time across every product page and the docs. These are the features that actually matter when you're wiring an agent to real email.

The multi-provider routing is the feature we'd point to first. Sending "through your own providers" means you keep your existing sender reputation and deliverability, and SendMux just orchestrates on top. That's a meaningfully different bet than a service that owns the sending IPs and asks you to trust its reputation.

How to Use SendMux

Getting an agent onto email is refreshingly short. The quickstart walks you through it in four moves:

1. Create a team. This is your isolation boundary. Every mailbox, provider, and API key lives under a team, so multi-tenant setups stay clean.
2. Add a mailbox. Spin up an inbox on myagent.mx to start, or verify your own domain for production.
3. Connect a provider. Point SendMux at SMTP, Gmail, Microsoft 365, or Amazon SES. Add more than one and set weights if you want load spreading and failover.
4. Send a test email. Fire a request over HTTP (or SMTP), subscribe to a webhook, and watch the delivery and inbound events flow in.

Two things stood out during setup. First, idempotency is built in — you can safely retry a mutating request without sending the same email twice, which matters a lot when an agent is looping. Second, webhook signatures use HMAC-SHA256, so you can verify events actually came from SendMux before your agent acts on them. These are the boring-but-critical primitives that separate real infrastructure from a weekend project.

If you're building on Claude or another MCP-aware agent, the Model Context Protocol servers are the fastest path — the agent discovers the tools and calls SendMux directly, no custom wrapper needed. The developer experience here is clearly designed for machines first, humans second, which is exactly right for this category.

Pricing Plans

This is where SendMux gets genuinely interesting. There are no plans in the traditional sense — no per-seat charge, no per-mailbox fee, no monthly subscription. You pay for what you route. That's a rare pricing model, and for agent workloads that spike unpredictably, it's the right one.

To put that in context: a million emails a month through your own providers works out to roughly $150. There's no credit card required to start, and because there's no per-mailbox fee, you can hand out thousands of agent inboxes without the pricing exploding. Compared to transactional platforms that charge $15–$100+/month the moment you cross a volume tier, this is a much friendlier curve for agent fleets that idle and then spike.

Why SendMux Stands Out

Best Alternatives

SendMux sits at the intersection of transactional email and agent infrastructure. Here's how it stacks up against the tools you'd otherwise reach for.

The short version: if you only need to send, Amazon SES or Resend are cheaper and simpler. The moment your agent needs to own an inbox, read replies, and route across providers with failover, SendMux is doing a job those tools weren't built for. It's the difference between an email sender and an email operating system for agents. If you're assembling a broader agent stack, it pairs naturally with the kind of automation platforms we cover in our AI tools directory.

Frequently Asked Questions

❓ What is SendMux used for?

It gives AI agents their own inboxes and a single control plane to send, receive, and process email. Agents read threads, send replies through your own providers, and react to inbound mail via webhooks or live events.

❓ How much does SendMux cost?

Usage-based: $0.15 per 1,000 emails through your own providers, $0.25 per 1,000 via managed Amazon SES, and $0.02 per GB of mailbox storage per month. No per-seat, per-mailbox, or subscription fees, and it's free to start with no credit card.

❓ Does SendMux work with Claude and other MCP agents?

Yes. SendMux ships Model Context Protocol servers, so MCP-aware agents like Claude can discover and call its tools directly without a custom integration layer.

❓ Does it replace my email provider?

No. SendMux sits on top of providers you already use — SMTP, Gmail, Microsoft 365, or Amazon SES — and handles routing, quotas, weighted delivery, failover, and inbound processing in one place.

❓ Which languages have SDKs?

Official SDKs cover TypeScript, Python, Go, PHP, and Ruby, plus a @sendmux/cli command-line tool for scripting and CI.

Final Verdict

4.9/5. SendMux is the rare tool that nails a real, specific problem instead of being one more email API. If you're building agents that need to live in an inbox — reading mail, sending replies, reacting to what lands — it removes a genuinely painful chunk of plumbing and does it with the kind of primitives (idempotency, signed webhooks, scoped keys, failover) you'd expect from mature infrastructure. The bring-your-own-provider model protects your deliverability, and the usage-based pricing is honest enough that spinning up thousands of agent inboxes doesn't wreck your budget.

If your agents touch email at all, SendMux is the first tool we'd try — and it's earned a spot next to the best agent-infrastructure tools we've reviewed this year.