Free API
Free source (opens in a new tab)
Month
For: Prototyping agents
Limits
- 2 mailboxes, 1 domain
- $1 usage credit
1 more
- 50 recipients per day; managed sending 2 per hour
Includes
- Email API
- MCP endpoint
1 more
- No credit card
API included
Productivity & writingDocumented overviewLast checked Sep 29, 2026
Sendmux: Pricing, Features & Best Use Cases
Email, calendar and file API that gives AI agents their own inboxes, with SDKs, CLI and an MCP endpoint.
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Our take: Choose Sendmux if you build AI agents that need their own inboxes, calendars and files through one API, with usage billing and no per-seat or per-mailbox fees.
Main limitation: The free plan has only 2 mailboxes and a $1 usage credit, its outgoing emails carry a Sendmux footer, and managed sending is limited to 2 per hour.
Who Sendmux fits
Choose Sendmux if you build AI agents that need their own inboxes, calendars and files through one API, with usage billing and no per-seat or per-mailbox fees. Based on the vendor's documentation, not on a hands-on test.
Best for: AI agent developers
Also a good fit for
- Teams building email automations
Consider another tool if free plan: 2 mailboxes, $1 usage credit
Or if
- Free outgoing emails carry a Sendmux footer
- Free managed sending limited to 2 per hour
Sendmux pricing and usage limits
Prices checked Sep 29, 2026 on the official US pricing page. Official pricing ↗ Prices and limits change often.
Free access and trial conditions
Free plan, no credit card, with $1 usage credit.
None beyond Free.
Paid plans: what changes as you upgrade
Pro API
$7 per team per month plus usage source (opens in a new tab)
Month
For: Production agents
Limits
- Unlimited* resources, fair usage applies
Includes
- Prepaid usage with top-ups
- No email footer
1 more
- Higher managed daily limits by review
API included
Enterprise API
Custom pricing source (opens in a new tab)
Unknown
For: High-volume senders
Limits
- Priced around your volume
Includes
- Dedicated infrastructure
- Priority support and custom SLAs
1 more
- Postpaid usage
API included
Usage caps, credits and extra costs
Connected outbound $0.50 per 1,000, managed $0.75 per 1,000, inbound $0.50 per 1,000, storage $0.02 per GB-month.
Not stated.
What is Sendmux, and how does it work?
Sendmux is an API that lets AI agents send and receive email, manage calendars and store files. It offers SDKs, a CLI, agent skills and a hosted MCP endpoint with OAuth. Billing is metered by usage rather than seats or mailboxes, with a free tier and a Pro team fee.
What you provide and what you get back
You provide
- API calls from agents or SDK/CLI
- Inbound email to agent mailboxes
- Calendar and file requests
You get back
- Sent emails
- Received and stored messages
- Calendar events and files
Where you can use it
- Platforms
- API
- Account
- Required
- Surfaces
- Email, calendar, files
- Connections
- SDKs, CLI, skills, hosted MCP
- Billing
- Per accepted recipient/delivery plus storage
- Free limits
- 2 mailboxes, 50/day
Best use cases for Sendmux
Scenarios are worked out by our editors from the documentation; the actual result depends on your input and plan.
Giving an agent an inbox
- Who
- AI agent developers
- You provide
- An agent skill or API key
- Expected result
- A dedicated mailbox the agent can read and send from
- Plan needed
- Free and up
- Limitation
- 2 mailboxes on Free source (opens in a new tab)
Sending transactional email from agents
- Who
- Teams automating notices
- You provide
- API send calls through managed or connected provider
- Expected result
- Delivered emails billed per accepted recipient
- Plan needed
- Pro
- Limitation
- Free adds a Sendmux footer source (opens in a new tab)
Inbound email processing
- Who
- Automation builders
- You provide
- Emails to a mailbox
- Expected result
- Messages stored and available to the agent
- Plan needed
- Pro for volume
- Limitation
- Inbound beyond $1 credit needs Pro source (opens in a new tab)
Connecting your own email provider
- Who
- Teams with existing SES or SMTP
- You provide
- Provider credentials
- Expected result
- Routing through your own account
- Plan needed
- Free and up
- Limitation
- Provider quotas still apply source (opens in a new tab)
What Sendmux does, and where it stops
Email API
Sends and receives email through an API and SDKs. Agents get real email.
Agent inboxes
Creates dedicated mailboxes for each agent. Isolates agent mail.
Calendar and file storage
Adds calendar and file storage to the same API. One surface for agent workspace.
Hosted MCP endpoint
Hosted MCP endpoint with OAuth, plus CLI and skills. Easy connection from agent apps.
Sendmux pros and cons
Where Sendmux is strong
- Free plan, no card required Prototype immediately documented source (opens in a new tab)
- No per-seat or per-mailbox fees Costs track usage only documented source (opens in a new tab)
- Documented MCP, SDK and CLI options Fits many agent stacks documented source (opens in a new tab)
- Billing is per event, not blocks of 1,000 Bills match actual sends documented source (opens in a new tab)
- Rejected recipients are not charged No cost for failed addresses documented source (opens in a new tab)
Limitations to check before paying
- Free plan: 2 mailboxes, $1 usage credit Small test scope documented source (opens in a new tab)
- Free outgoing emails carry a Sendmux footer Not suitable for real customers documented source (opens in a new tab)
- Free managed sending limited to 2 per hour Slow for any real sending documented source (opens in a new tab)
- Postpaid usage only on Enterprise Pro must prepay credit documented source (opens in a new tab)
- Unlimited on Pro is subject to fair usage and approval Large increases need pre-approval documented source (opens in a new tab)
How to use Sendmux for giving an AI agent its own inbox
Set up Sendmux
- Create a free account at sendmux.ai
- Create a team and add a domain or use the default
- Create a mailbox for the agent
- Install the skills with npx skills add Sendmux/skills or connect the hosted MCP endpoint
Example: giving an AI agent its own inbox
- Send a test email from the agent
- Reply to it to trigger an inbound delivery
- Read the message through the API
- Check usage in the dashboard
Review and export the result
- Confirm delivery in the recipient inbox
- Check the usage credit decreased as expected
Check the agent inbox for new messages from customers and draft a reply to each.
Sendmux API
API access and separate billing
Sendmux has no public API in its official documentation. SDKs, CLI, skills and hosted MCP at mcp.sendmux.ai API documentation ↗
Privacy, data use and commercial rights
Summarized from the vendor's current policies; this is not legal advice and a vendor's badge is not an independent audit.
How your inputs and outputs are handled
Outbound body content is held temporarily during delivery; mailbox content is stored until you delete it.
Does not use email content or metadata to train ML models.
Outbound bodies deleted after delivery; mailbox data until customer deletes it.
Commercial use and plan restrictions
Not stated.
Dedicated infrastructure on Enterprise.
Controls for business users
- AES-256-GCM encrypted credentials
Policies: sendmux.ai/privacy/
Other tools to consider instead of Sendmux
"Better suited" is based on documented features and pricing, not on a head-to-head test.
Zapier: better suited to
Make: better suited to
Explore Sendmux further
Sendmux pricing, plan by planEvery plan with USD prices, limits and what is free.Best Sendmux alternativesTools for the same job, by need and starting price.Best AI writing and productivity toolsOur picks in this category, compared by price and free tiers.Prompts for SendmuxReady-to-copy prompts with their sources.
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Sendmux FAQ
What does the Free plan include?
2 mailboxes, 1 domain, a $1 usage credit and 50 provider-accepted recipients a day; outgoing emails carry a Sendmux footer. source (opens in a new tab)
What does Pro cost?
$7 per team per month plus usage: $0.50 per 1,000 connected outbound, $0.75 per 1,000 managed, $0.50 per 1,000 inbound and $0.02 per GB-month storage. source (opens in a new tab)
Is usage billed in blocks?
No, billing counts each event, so 1,400 sent recipients bills as 1,400. source (opens in a new tab)
Does Sendmux train on my email?
No. The privacy policy says email content and metadata are not used to train ML models. source (opens in a new tab)
How do agents connect?
Through SDKs, a CLI, agent skills or the hosted MCP endpoint with OAuth. source (opens in a new tab)
Sources and review methodology
Official sources and verification dates
- Sendmux home homepage · checked Oct 1, 2026
- Sendmux pricing pricing · checked Oct 1, 2026
Not verified: Calendar and file limits; US availability.
How we documented and maintain this page
Every fact about Sendmux above comes from the 2 official pages listed here, last compared on Sep 29, 2026; we have not run Sendmux ourselves, so it has no editorial score. How we check and re-check pages.
Article updated Oct 10, 2026; prices checked Sep 29, 2026. Outbound links to vendors are plain links; we do not earn commissions. How we review · Report an error