Extend a StableEmail forwarding inbox by 365 days for $8 USDC via micropayment — no API keys or accounts.
What it does
This endpoint extends an existing StableEmail forwarding inbox (`username@stableemail.dev`) by 365 days for a one-time payment of $8 USDC, representing a 34% discount compared to twelve monthly top-ups at $1 each. 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. Payment is handled through the x402/MPP protocol using USDC on Base (eip155:8453), Solana, or Tempo.
The endpoint accepts a POST request with a JSON body containing the `username` of the inbox to extend. Anyone can top up any inbox — no wallet ownership proof (SIWX) is required, only the micropayment. On success, the response includes the updated expiration date, total days remaining, and confirmation that 365 days were added. The inbox must already exist (purchased via `/api/inbox/buy`).
StableEmail's broader platform includes sending email from a shared relay address ($0.02/email), purchasing custom subdomains ($5 one-time), sending from subdomains or inbox addresses ($0.005/email), creating per-address inboxes on subdomains ($0.25), and reading received messages programmatically ($0.001/query). Inboxes support forwarding to a real email address, programmatic message retention (90-day content retention, 500 message cap), or both. Cancellation returns a pro-rata USDC refund on-chain. Agent integration is available via the agentcash MCP server or CLI.
Capabilities
Use cases
- —Prepaying a full year of email forwarding for a project or alias address
- —Agents autonomously maintaining inbox subscriptions without human intervention
- —Gifting or sponsoring inbox time for another user's forwarding address
- —Bulk-discounted inbox renewal for long-running services or bots
- —Maintaining a pseudonymous email address tied to a crypto wallet
Fit
Best for
- —AI agents that need persistent email forwarding without account management
- —Developers wanting a simple, accountless forwarding inbox with annual billing
- —Crypto-native users who prefer USDC micropayments over traditional subscriptions
Not for
- —Users who need to create a new inbox (use /api/inbox/buy instead)
- —High-volume transactional email sending (this endpoint only extends inbox duration, not sending)
- —Users without a crypto wallet or USDC on Base/Solana/Tempo
Quick start
# Extend inbox 365 days ($8 USDC) via agentcash CLI
npx agentcash fetch POST https://stableemail.dev/api/inbox/topup/year \
--body '{"username": "alice"}'Example
Request
{
"username": "alice"
}Response
{
"inbox": "alice",
"success": true,
"daysAdded": 365,
"expiresAt": "2026-06-15T00:00:00.000Z",
"daysRemaining": 380
}Endpoint
Quality
Full OpenAPI 3.1 schema with detailed x-guidance documentation, clear pricing ($8 USDC), and well-defined request/response structures. The probe returned 405 because the endpoint only accepts POST (not HEAD/GET), which is expected behavior — the endpoint is live based on the 200 root status and comprehensive OpenAPI spec. Minor deduction for no captured 402 challenge on this specific endpoint and no dedicated docs page (404).
Warnings
- —Probe returned 405 because HEAD/GET were tried but endpoint only accepts POST — this does not indicate the endpoint is down
- —No dedicated documentation page exists (/docs returns 404); all docs are in the OpenAPI x-guidance field and landing page
- —Inbox must already exist before top-up; calling this on a non-existent username will fail
Citations
- —Top up inbox 365 days costs $8 USDC with 34% savingshttps://stableemail.dev
- —Anyone can top up any inbox — no SIWX requiredhttps://stableemail.dev/.well-known/x402
- —Payment accepted on Base (eip155:8453), Solana, or Tempo in USDChttps://stableemail.dev
- —Cancel anytime for a pro-rata refundhttps://stableemail.dev
- —Agent integration via agentcash MCP server or CLIhttps://stableemail.dev