Email API built for AI agents.

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June 26, 2026

Sendmux

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Email API built for AI agents.

AI Email API One-Platform Solution Provider Routing Flexibility Routing Control Email Delivery Events Outbound Mailsfrom $0.15

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Overview

Sendmux is an Email API primarily designed to cater for AI agents, offering features such as agent inboxes, outbound routing via user-designated providers and automatic failover.

It provides a unified API surface that simplifies tasks related to outbound mails, agent mailboxes, routing control and delivery events. One of its key features allows users to create agent-shaped mailboxes either on verified domains or shared agent-specific domains.

The API ensures that raw message contents are available upon request and facilitates tasks like stripping signatures and quoted history, optionally making links, headers, and attachments available.For outbound emails, Sendmux allows users to break free from provider lock-ins, enabling sending from a mailbox over HTTP or SMTP protocols.

It also ensures safe HTTP retries through Idempotency-Key support. The platform also incorporates messaging routing considerations, allowing users to bind a mailbox key to all active providers, selected providers or delivery groups and implement control measures such as provider weights and quotas to manage traffic.Sendmux also eradicates the need for polling loops through use of signed webhooks for server delivery, an SSE stream for real-time mailbox clients, and typed change polling when a client needs to resync its state.Ultimately, Sendmux offers a one-platform solution, helping you coordinate services that were previously disjointed, ensuring the AI agents focus on the content rather than parsing formatting.

It also provides provisions for flexibility, enabling you to bring your own providers or route across multiple accounts.

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June 26, 2026

Initial release of Sendmux.

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Author

Roshan Jonnalagadda

@roshan-jonnalagadda

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Joined: June 2026

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Pros and Cons

Pros

Agent-shaped mailboxes

Unified API surface

Outbound routing flexibility

Automatic failover

Raw message content access

Strip signatures and quoted history

HTTP or SMTP protocols support

Idempotency-Key for HTTP retries

Mailbox key binding

Provider weights and quotas

Eliminates need for polling loops

Signed webhooks for server delivery

SSE stream for real-time updates

Typed change polling

Option to bring your own providers

Eliminates provider lock-in

Covers services previously disjointed

Allows provider routing across multiple accounts

Agent inbox creation

Clean JSON format emails

Strips HTML noise from incoming mails

Supports mail control via API

Dedicated mailboxes for each agent

Unified inbox for all agents

Supports keyboard-driven view

Inbox search across the fleet

View 21 more pros

Cons

Lacks explanatory documentation

No multi-language support

Complex setup process

UI not user-friendly

No integration guidance

Absence of trial version

Cost per email sent

No dedicated support channel

Absence of offline capabilities

No customization options

View 5 more cons

Q&A

What is Sendmux?

Sendmux is an Email API built to cater for AI agents. This platform is primarily designed to unify tasks like outbound mails, agent mailboxes, routing control, and delivery events. It allows users to create agent-shaped mailboxes on verified domains or shared agent-specific domains. It also enables users to free themselves from provider lock-ins by allowing the use of mailbox over HTTP or SMTP protocols to send mails.

How does Sendmux work?

Sendmux works by offering a unified API surface that simplifies tasks related to outbound mails, agent mailboxes, routing control, and delivery events. It allows users to create agent-shaped mailboxes, which can be created on verified domains or shared agent-specific domains. Sendmux supports both HTTP and SMTP protocols for outbound emails and allows users to break free from provider lock-ins. It ensures safe HTTP retries through Idempotency-Key support. Sendmux also continuously updates its user's status through server delivery using signed webhooks, an SSE stream, and typed change polling.

What is the key feature of Sendmux?

A key feature of Sendmux is its ability to create agent-shaped mailboxes on either verified domains or shared agent-specific domains. It provides a unified API surface that simplifies tasks related to outbound mails, agent mailboxes, routing control, and delivery events. Furthermore, Sendmux not only ensures that the raw message contents are available upon request but also facilitates tasks like stripping signatures and quoted history, optionally making links, headers, and attachments available.

How does Sendmux handle outbound emails?

For outbound emails, Sendmux enables users to break free from provider lock-ins, enabling sending from a mailbox over HTTP or SMTP protocols. With its Idempotency-Key support, Sendmux ensures safe HTTP retries. It incorporates considerations into the messaging routing, allowing users to bind a mailbox key to all active providers, selected providers, or delivery groups, thereby implementing control measures such as provider weights and quotas to manage traffic.

How are agent-shaped mailboxes created in Sendmux?

In Sendmux, users can create agent-shaped mailboxes on either verified domains or shared agent-specific domains. Users have the flexibility to customize their domains to better manage and segregate their emails tailored to their respective needs.

Does Sendmux allow raw message contents to be available upon request?

Yes, Sendmux ensures that raw message contents are made available upon request. This feature enhances the user's control over how they receive and manage their emails, ensuring that no crucial information is overlooked or lost in the process.

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What tasks does Sendmux facilitate in relation to emails?

Sendmux facilitates a multitude of tasks related to emails. Aside from making raw message contents available upon request, it aids in tasks like stripping signatures and quoted history. It also has the capability to make links, headers, and attachments available, increasing the adaptability and relevance of sent messages according to the users' preferences.

What are the routing considerations Sendmux incorporates for messaging?

Sendmux incorporates several routing considerations into its messaging structure. It allows users to bind a mailbox key to all active providers, selected providers, or delivery groups. Further, it lets users implement controls like provider weights and quotas to effectively manage traffic.

How does Sendmux eradicate the need for polling loops?

Sendmux eradicates the need for polling loops by utilizing signed webhooks for server delivery, an SSE stream for real-time mailbox clients, and typed change polling for clients that need to resynchronise their state. These features work together to provide a seamless experience where continuous polling loops are now a thing of the past.

Can Sendmux allow me to bring my own email providers?

Yes, Sendmux enables you to bring in your own email providers. It provides the users with the flexibility to choose their preferred providers, promoting greater control over how they send their emails and manage their communications.

What flexibility does Sendmux offer in terms of provider routing?

Sendmux offers substantial flexibility in terms of provider routing. The system allows users to break free from provider lock-ins and send from a mailbox over either HTTP or SMTP protocols. It also gives users the freedom to bring their own providers or route across multiple accounts, ensuring adaptability and customization in accordance with their personal or business communication requirements.

How does Sendmux's Idempotency-Key support ensure safe HTTP retries?

Sendmux employs Idempotency-Key support to ensure safe HTTP retries. This feature ensures that commands meant to be executed only once do not inadvertently get executed multiple times due to network issues. This supports the reliability and accuracy of Sendmux's email services.

What are the specifics of Sendmux in terms of server delivery, SSE stream and change polling?

For server delivery, Sendmux utilises signed webhooks, eliminating the need for polling loops. It uses an SSE stream for real-time mailbox clients ensuring immediate communication without delay. In instances where a client needs to resynchronise state, Sendmux provides typed change polling.

What are the advantages of using Sendmux's unified API surface?

Using Sendmux's unified API surface provides several advantages, including simplifying tasks related to outbound mails, agent mailboxes, routing control and delivery events. It further helps users coordinate several services that were previously disjointed, allowing AI agents to focus on the content rather than parsing formatting. Ultimately, Sendmux's unified API surface helps streamline users' operations in an efficient and manageable manner.

How are agent mailboxes managed in Sendmux?

In Sendmux, Agent mailboxes are managed through a unified API interface that allows users to perform a variety of tasks easily. This includes creating agent-shaped mailboxes in verified domains or shared agent-specific domains, leveraging provider weights and quotas for traffic management, and obtaining raw message contents upon request.

How does Sendmux handle tasks related to outbound mails, agent mailboxes, routing control, and delivery events?

Sendmux provides a unified API surface that simplifies tasks related to outbound mails, agent mailboxes, routing control, and delivery events. It allows users to create agent-shaped mailboxes and enables sending from a mailbox over either HTTP or SMTP protocols. It incorporates messaging routing considerations and ensures safe HTTP retries through Idempotency-Key support. Sendmux also uses signed webhooks for server delivery, an SSE stream for real-time mailbox clients, and typed change polling when a client needs to resynchronize its state.

How does Sendmux handle automatic failovers?

Sendmux handles automatic failovers by providing outbound routing through user-designated providers. In case one provider faces issues, Sendmux ensures that service continues with minimal disruption, automatically rerouting communications via other active providers. This feature ensures uninterrupted service under any circumstances.

How does Sendmux simplify outbound mails?

Sendmux simplifies outbound mails by allowing users to send from a mailbox over HTTP or SMTP protocols, thereby breaking free from provider lock-ins. It also ensures safe HTTP retries through Idempotency-Key support. These features streamline the sending process, making it easier and more flexible for users.

How does Sendmux offer control measures for traffic management?

Sendmux offers several control measures to manage traffic. It allows users to implement provider weights and quotas, enabling them to maintain traffic within set boundaries. Users can bind a mailbox key to all active providers, selected providers, or delivery groups, offering greater control and flexibility in distributing and managing their emails.

What does Sendmux mean by AI agents focusing on content rather than parsing formatting?

In Sendmux, AI agents can focus on the content rather than parsing formatting due to its feature of making raw message contents available upon request. As it facilitates tasks like stripping signatures and quoted history, AI agents can concentrate on the content and intent of the messages, thereby ensuring a more effective and targeted communication strategy.

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