Extend a StableEmail forwarding inbox by 90 days for $2.50 USDC via micropayment — no API keys or accounts.
What it does
This endpoint extends an existing StableEmail forwarding inbox (`username@stableemail.dev`) by 90 days for a one-time payment of $2.50 USDC, representing a 17% discount over the monthly $1/30-day rate. It is part of the StableEmail platform, which provides pay-per-send email delivery, forwarding inboxes, and custom subdomains — all accessible via single HTTP requests with no API keys or accounts required. Payments settle via the x402/MPP protocol using USDC on Base (eip155:8453), Solana, or Tempo.
The request body requires only a `username` field identifying the inbox to top up. Anyone can top up any inbox — no wallet ownership proof (SIWX) is required, making it suitable for gifting or automated renewal by agents. The endpoint returns the updated expiration date and total days remaining. The inbox itself supports email forwarding to a real address, programmatic message reading via API, and outbound sending from the inbox address.
StableEmail is designed for AI agent integration. The recommended client is agentcash-skills (`npx -y agentcash@latest`), which handles the 402 payment negotiation automatically. Any x402/MPP-compatible client paying USDC will also work. The platform also offers 30-day top-ups ($1), yearly top-ups ($8, 34% discount), inbox cancellation with pro-rata refunds, and custom subdomain purchases ($5).
Capabilities
Use cases
- —Automatically renewing a forwarding inbox for an AI agent's email address on a quarterly basis
- —Gifting or sponsoring inbox time for another user without needing their wallet credentials
- —Maintaining a persistent email identity (username@stableemail.dev) with discounted bulk pricing
- —Programmatic inbox management where an agent tops up its own mailbox before expiry
Fit
Best for
- —AI agents needing a persistent email address with automated renewal
- —Users wanting a cheap forwarding inbox with quarterly billing
- —Developers integrating email capabilities without traditional API key management
- —Privacy-conscious users wanting a forwarding address decoupled from their real email
Not for
- —High-volume transactional email sending (this endpoint only extends inbox duration, not sends)
- —Users who need a full-featured email client with folders, search, and rich UI
- —Organizations requiring custom domain email (use the subdomain endpoints instead)
Quick start
# Top up an inbox for 90 days ($2.50 USDC)
curl -X POST https://stableemail.dev/api/inbox/topup/quarter \
-H 'Content-Type: application/json' \
-d '{"username": "alice"}'
# Returns 402 → pay with any x402/MPP client → resend with payment headerExample
Request
{
"username": "alice"
}Response
{
"inbox": "alice",
"success": true,
"daysAdded": 90,
"expiresAt": "2025-10-15T00:00:00.000Z",
"daysRemaining": 120
}Endpoint
Quality
Full OpenAPI 3.1 schema with detailed x-guidance documentation, clear pricing ($2.50 USDC), and well-defined request/response shapes. The endpoint returned 405 on HEAD/GET probes because it only accepts POST, which is expected behavior — the OpenAPI spec confirms it is a POST endpoint. No dedicated docs page exists (404), but the llms.txt guidance and OpenAPI spec are comprehensive.
Warnings
- —Probe returned 405 because HEAD/GET were tried on a POST-only endpoint; the endpoint is expected to be live when called with POST and a valid payment header.
- —No dedicated /docs page exists (returns 404); all documentation is in the OpenAPI spec and llms.txt guidance.
Citations
- —Top up inbox 90 days costs $2.50 USDC (save 17%)https://stableemail.dev
- —Anyone can top up any inbox — no SIWX requiredhttps://stableemail.dev
- —Payment settles via x402/MPP using USDC on Base, Solana, or Tempohttps://stableemail.dev
- —Request body requires only a username fieldhttps://stableemail.dev/.well-known/x402
- —Cancel anytime for a pro-rata refundhttps://stableemail.dev
- —Recommended agent integration via agentcash-skills (npx -y agentcash@latest)https://stableemail.dev