RECEIPT RELAY · REQUEST AND EVENT FLOW
Browser
Validated receipt form
Short delivery polling
No provider secrets
FastAPI
Render HTML + text
Keep API key server-side
Verify webhook HMAC
SMTPFast
Queue and relay email
Expose per-email trace
Sign delivery webhooks
Inbox
Recipient server
Accept or reject
Open and click
POST receipt
POST /emails
SMTP relay
SIGNED WEBHOOK · RAW-BODY HMAC
GET /emails/:id · SHORT UI POLLING
FastAPI owns the application boundary. SMTPFast owns the email pipeline. A production webhook consumer persists events durably.