Transactional Messaging APIs for AI Agents
Pay-per-send delivery with disposable inboxes, custom subdomains, and message retention — no API keys or accounts required.
npx agentcash try https://stableemail.dev
How It Works
Make a single POST request to send a message through a shared relay address for $0.02 per send. No registration, no API keys — payment is handled inline via the x402 or mpp protocol from your AgentCash wallet.
For higher volume or branding needs, purchase a custom subdomain on stableemail.dev for $5, then send messages from that subdomain at $0.005 each. You control the sender identity without managing DNS or SMTP infrastructure.
Programmable Inboxes
Buy an inbox for $1 (30-day period) and receive messages programmatically. Each inbox retains message content and attachments for 90 days. List messages for $0.001 per call and read individual messages — including text, HTML, and temporary attachment download URLs — for the same price.
Subdomain inboxes work the same way at $0.25 each, with a cap of 100 inboxes and 500 messages per inbox. Both inbox types support forwarding or pure programmatic access.
Top up inbox subscriptions monthly ($1), quarterly ($2.50, save 17%), or annually ($8, save 34%). Cancel anytime and receive a pro-rata refund for unused time.
Subdomain and Inbox Management
Check subdomain and inbox status, update settings, and manage authorized signers through free authenticated endpoints using wallet-based signing (SIWX).
Delete individual messages, remove inboxes, and list all inboxes under a subdomain — all at no additional cost beyond the initial purchase.
Use cases
AI agent outbound communication
Give autonomous agents the ability to send transactional messages on demand. Each send is a single API call with inline payment — no credential provisioning or OAuth setup needed.
Disposable inboxes for sign-up verification
Spin up temporary inboxes to receive verification codes during automated account creation flows. Read the message content programmatically and extract tokens.
Customer notification pipeline
Send order confirmations, status updates, or alerts from a branded subdomain at $0.005 per message, with no monthly minimums or committed volume.
Inbound message processing
Create programmatic inboxes that collect incoming messages. Poll the messages endpoint, parse HTML or text bodies, and route content to downstream systems.
Multi-tenant messaging under one subdomain
Purchase a subdomain once, then create up to 100 inboxes for different tenants or workflows. Manage each inbox independently with its own settings and message store.
Endpoints
Frequently asked questions
What authentication method does this API use?
Paid endpoints use the x402 and mpp protocols — payment is sent inline with each request from your AgentCash wallet. No API keys or account creation required. Free management endpoints use wallet-based signing (SIWX) to verify ownership.
How much does it cost to send a message?
Sending via the shared relay address costs $0.02. If you own a custom subdomain ($5 one-time purchase), sending from that subdomain or its inboxes costs $0.005 per message. Reading and listing inbox messages costs $0.001 per call.
How long are inbox messages retained?
Message content and attachments are retained for 90 days from the date of receipt. Attachment download URLs expire after 1 hour but can be re-generated by reading the message again.
Can I receive messages programmatically?
Yes. Purchase an inbox ($1 for 30 days) or create a subdomain inbox ($0.25) with retainMessages enabled. Then list and read messages via the API. Each inbox holds up to 500 messages.
What happens if I cancel an inbox?
You receive a pro-rata refund for the unused portion of the subscription period. The inbox and its messages are removed upon cancellation.
Is there a rate limit or sending cap?
The API documentation does not specify hard rate limits. Subdomain inboxes are capped at 100 per subdomain with 500 messages each. For sending, you pay per request with no stated monthly minimum or maximum.
This page is compiled from live OpenAPI specs published by each API provider. Content last reviewed Apr 7, 2026 by the AgentCash team. How we build these pages.