Sendmux
Email Inbox and Sending API for AI Agents
Developer Tools·AI & Machine Learning·Productivity·Marketing·Communication
About
Sendmux is one workspace and API to send, receive, route, and monitor email, built for products and AI agents.
Most email platforms stop at sending. Sendmux gives your app or agent a real inbox. It can send mail, receive it, hold it, reply, and search across threads, all from the same place.
Built for AI agents
This is where Sendmux is different. An agent can self-register for its own constrained mailbox, then invite you to approve it. Until you say yes, it can read and receive but cannot send. Human approval is the default, not an add-on. So you can hand an agent its own email address without handing over your sending reputation.
Key features
- Send single or batch transactional email through a REST API
- Real mailboxes on your own verified domain or the shared
myagent.mxdomain - Full inbox access to read, triage, reply, and sync messages
- Delivery logs and metrics you can search and export to CSV
- Signed webhooks for delivery and inbound events
- Scoped API keys for sending, mailbox, and management
- Connect how you work: hosted MCP with OAuth, a CLI, and SDKs for Python, Go, Ruby, PHP, and TypeScript
- 50 mailboxes per team to start
Who it's for
- Developers who want transactional sending plus a programmable inbox in one API
- Teams building AI agents that need to send and receive email safely
- Anyone tired of gluing a sending service onto a separate receiving service
Give an agent its own inbox, keep a human in the loop on every send, and see exactly what went out. That's Sendmux.
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FounderPlaybooks.
What other founders did to grow.
2722 dispatches from hundreds of founders, pulled from the week's best podcasts.
there was a lot of anxiety when I tapped out my network I was like what if this is just a business where people I call and they like me or they know me will buy it and then it stops
When Referrals From Strangers Start Arriving, Your Business Is Real
After signing his first customers through personal relationships, Ian hit a defining moment of doubt: was this a real business or just goodwill from friends? The anxiety resolved only when referrals started coming in from strangers. This transition — from network sales to word-of-mouth from people you've never met — is the real validation signal founders should watch for.
Definitely making sure that there's no paywall with that is important if you want to make hay off of organic or viral. The only ones I've ever had be successful are the ones that are like core to the product — you have to think about it early.
Viral sharing features must be free and core to the product — paywalling them kills the loop
Every share-to-TikTok, export, or social feature that sits behind a paywall breaks the viral loop before it starts. One Second Every Day's monthly giveaway — where users share their month video for a chance to win — works because sharing is frictionless and free. The product insight: viral mechanics need to be designed into the core product early, not bolted on later as premium add-ons.
There's a play for whatever you're stuck on.
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Comments
1Hey 👋 I'm Roshan, founder. Sendmux gives your app or AI agent a real inbox: send, receive, reply. But agents can't send until you approve. Try it: make a mailbox, send a test, watch the log.