VaultSandbox

VaultSandbox is a self-hosted, receive-only SMTP testing gateway designed to replicate how email actually behaves in production environments. Unlike traditional email testing tools that simply answer "did my app send an email?", VaultSandbox validates the complete email delivery pipeline—including TLS negotiation, SPF/DKIM/DMARC authentication, spam scoring via Rspamd integration, and protocol-level SMTP behavior.

The tool is built for developers and QA teams who are tired of email tests passing locally but failing in production. It offers built-in chaos engineering capabilities that let you inject latency, simulate greylisting (451 retries), drop connections, return specific SMTP errors, or blackhole messages entirely—all configurable per inbox.

VaultSandbox offers three deployment modes: zero-config localhost for quick local development, automatic domain provisioning via vsx.email for instant public DNS and TLS, or full custom domain support for complete control. It runs entirely in your infrastructure—laptop, CI runner, VM, Kubernetes, or even air-gapped environments—with no shared inboxes or vendor lock-in.

For CI/CD integration, VaultSandbox provides first-class SDKs for Node.js, Python, Go, .NET, and Java with SSE-based deterministic waits—eliminating the need for polling loops or arbitrary sleep statements that cause flaky tests. The CLI and Web UI provide real-time email inspection, attachment handling, link extraction, and authentication visualization.

Security-conscious teams can enable optional per-inbox zero-knowledge encryption where emails are encrypted immediately on receipt and private keys never leave the client. The core gateway is open source under Apache 2.0 and free forever.