check-if-email-exists is a command-line tool and library written in Rust that verifies whether an email address exists without sending any email, and includes an HTTP backend for integration into applications.
The tool solves the problem of email validation by performing SMTP checks directly against mail servers rather than relying on pattern matching or sending test messages. It connects to the recipient's mail server and queries whether the address is valid, returning structured JSON output that indicates existence status along with diagnostic details. The approach works across three deployment models: as a standalone CLI binary that checks email directly from your computer, as a Dockerized HTTP backend service that can run on a cloud instance or private server, or as a Rust library for programmatic use within other Rust projects.
Developers should choose this tool if they need reliable email verification without the overhead of sending actual messages, particularly when processing bulk email lists or validating user input during account creation. The HTTP backend deployment suits teams running infrastructure on their own servers or cloud instances, though outbound SMTP port 25 must be open. For high-volume operations, the README notes that SMTP proxy servers become necessary to avoid rate limiting and IP reputation issues. The CLI is appropriate for one-off verification tasks or local development, while the library integration works best for Rust-based applications that need email validation as an embedded feature.
The project maintains active development with regular updates to handle edge cases and improve reliability across different mail server implementations. The codebase shows attention to practical deployment concerns, including explicit documentation of network requirements and guidance on scaling to production volumes through proxy services. The tool includes a live demo and offers a commercial SaaS alternative for users who prefer managed hosting, indicating the maintainers understand both open-source and commercial use cases.
