Sendmux is an email API and inbox service built specifically for AI agents and agent platforms. It gives agents real inboxes on shared or custom domains, handles sending and receiving across multiple providers, and returns messages as clean JSON instead of heavy HTML. Teams building support bots, sales agents, schedulers, and monitoring agents use it to add controlled, production email without wiring together separate SMTP, inbox, and routing tools.
Key Features:
Agent-shaped inboxes: Create dedicated mailboxes on the shared myagent.mx domain or a verified domain and read messages as structured JSON with signatures, quoted history, links, headers, and attachments handled for agents.
Unified inbox dashboard: View every agent inbox in a single, keyboard-driven UI, search across mailboxes, label threads, act in bulk, and let human operators jump into any conversation.
Provider-agnostic outbound routing: Connect Gmail, SendGrid, Mailgun, Resend, or any SMTP provider, then use weighted routing, per second to per day quotas, and automatic failover so outages or limits do not stall agents.
Multi-tenant teams and scoped keys: Run many customers on one account using teams, roles, and up to 100 active API keys per team, with root, send-only, and mailbox-scoped keys that keep people and services separated.
Live events via webhooks and SSE: Consume HMAC-signed webhooks with 24 hour retries or Server-Sent Events streams for message.received, message.delivered, and spam events, all without polling loops.
AI-native tooling and MCP server: Plug agents in through an MCP server, TypeScript and Python SDKs, a CLI, LangChain toolkit, and an AI SDK integration so LLM agents can call send_email, list_messages, and reply as normal tools.
Pros
Built for agents, not humans: Email bodies arrive as compact JSON with signatures and quoted threads stripped; in the example, a 3,247 token email is reduced to 127 tokens, saving about 96 percent of tokens for the model.
Strong provider flexibility: Multi-provider routing with weights, quotas, and automatic failover gives teams control over deliverability and compliance while keeping one API surface.
Cost structure that matches agents: Usage billing at $0.15 per 1,000 emails with no per-seat or per-mailbox fees fits fleets of thousands of lightweight agent inboxes.
Platform-ready multi-tenancy: Teams, tenant-scoped usage records, and granular keys make it practical for SaaS platforms to run email for many customers from one Sendmux deployment.
Developer-focused docs and examples: Inline cURL, JavaScript, Python, and agent prompt examples shorten the path from idea to a working AI email integration.
Cons
Email-focused scope: Product scope is email infrastructure, so multi-channel campaigns across SMS, push, or chat still require additional tools.
Developer-first experience: Setup assumes comfort with APIs, keys, webhooks, and SDKs, which keeps non-technical teams reliant on engineers.
No WebSockets yet: Live inbox streams use Server-Sent Events, while WebSockets are described as a roadmap feature rather than available today.
Who is Using Sendmux?
AI agent builders: Give each support, sales, or workflow agent its own inbox so it can read threads, reply in place, and follow up autonomously under owner control.
SaaS platform teams: Embed email for customers’ agents using multi-tenant teams, scoped keys, and tenant-level usage and billing.
Customer support leaders: Run AI triage for support inboxes while keeping a unified view and human override in the Sendmux dashboard.
Sales and growth engineers: Let outbound and follow-up agents send through shared domains and multiple providers while routing around rate limits.
Uncommon Use Cases: Scheduling agents that confirm bookings and reschedule over email; internal monitoring agents that route alerts and receipts to operations teams.
Pricing:
Pricing Model: Sendmux is purely usage-based with no seats, mailboxes, tiers, or minimums; free to start with no credit card required
Standard Email: $0.15 per 1,000 emails sent through your own connected providers or received
Managed Amazon SES: $0.25 per 1,000 emails sent through the built-in Amazon SES account
Mailbox Storage: $0.02 per GB per month across all team mailboxes
Disclaimer: Please note that pricing information may not be up to date. For the most accurate and current pricing details, refer to the official Sendmux website.
What Makes Sendmux Unique?
Sendmux treats AI agents as first-class email users and wraps everything they need into one tenant-aware API: agent-shaped inbox parsing, multi-provider routing with quotas and failover, HMAC-signed events, and scoped keys for both humans and services. That combination lets platforms spin up thousands of inboxes on their own domains, keep strict control over who can send or read, and still process more than 10 million inbound emails per day at sub-second latency according to its own figures.
How We Rated It:
Accuracy and Reliability: 4.4/5
Ease of Use: 3.8/5
Functionality and Features: 4.3/5
Performance and Speed: 4.6/5
Customization and Flexibility: 4.1/5
Data Privacy and Security: 4.2/5
Support and Resources: 3.7/5
Cost-Efficiency: 4.5/5
Integration Capabilities: 4.0/5
Overall Score: 4.2/5
Email Infrastructure Shaped For AI Agents:
Sendmux gives AI and platform teams a focused way to run real email for agents without building their own inbox, routing, and multi-tenant security stack. It shines when many small agents need tightly controlled access to sending and reading on production domains, with provider choice and signed events built in. It is less suited to non-technical marketers or multi-channel outreach, but for agent-centric email infrastructure it covers the hard engineering problems in a compact, developer-friendly service.
