Email Threat Modeling for SaaS: Preventing Compromised API Key Abuse

The Real-World Blast Radius of a Leaked Email API Key

Email API credentials are prime targets for automated credential scrapers. Automated scanners continuously crawl public GitHub repositories, Docker container registries, and paste sites. Within 90 seconds of an API key being pushed to a public branch, automated botnets begin utilizing the key to dispatch hundreds of thousands of cryptocurrency phishing, malware, or spam campaigns.

Because the sending domain has valid SPF, DKIM, and DMARC authentication configured by your engineering team, these spam campaigns achieve high inbox placement initially. Within 4 hours, however, the mailbox providers react: your domain is placed on Spamhaus and Barracuda blacklists, Google Postmaster reputation collapses to "Bad", and all legitimate customer transactional emails (password resets, invoices) land in spam for weeks.

Defense in Depth: The 4-Tier Security Engineering Architecture

How SadaSend Per-Key Recipient Allowlists Eliminate the Threat

Traditional email providers (SendGrid, Resend, Mailgun) operate under an all-or-nothing security model: if an API key has sending permissions, it can email anyone in the world. A compromised developer key or rogue AI agent loop has an infinite blast radius.

SadaSend introduces per-key recipient allowlists enforced directly at the edge API gateway. You can restrict staging and agent keys to specific domains (*@yourcompany.com) or specific developer addresses. If an attacker or looping agent attempts to send outside the allowlist, the API immediately rejects the request with HTTP 403 before a single message enters the outbound queue.

{
  "error": "Forbidden",
  "status": 403,
  "code": "RECIPIENT_ALLOWLIST_VIOLATION",
  "message": "Recipient 'victim@external-domain.com' is not permitted by API key allowlist constraint: '*@yourcompany.com'"
}