SMTP Cracker (KGB UNIT / KGB Legacy, v1.0) is a professional SMTP authentication and mailbox-testing tool.

It checks SMTP, IMAP (993/143 SSL/TLS), and POP3 (995/110 SSL/TLS) logins worldwide, with password brute-force (smart patterns + common passwords), adaptive rate limiting, fast-fail on network errors, DNS negative cache, connection pooling, and async logging. It targets 60+ Japan SMTP providers plus major transactional email services, with graceful shutdown on signals.