Real-Time Mailbox Email Verification -- 2

I’m building a workflow that must confirm whether an address is truly deliverable the moment it is entered. Simple syntax or domain checks aren’t enough—I need full mailbox-level verification executed in real time, with the result returned fast enough to block bad data from ever reaching my database.

When an address fails validation the system should immediately notify the user, either directly in the UI or through a clear, structured API response that I can surface in the frontend. No message should be silently rejected or left unacknowledged.

Core expectations
• Perform SMTP-level mailbox verification for every submission.
• Average response time should stay well under one second for a single lookup.
• The component must be callable both via REST and as an internal function so I can wire it into multiple services.
• Clean, well-documented code plus a short setup guide so my team can deploy it on our Linux environment.

Acceptance criteria
1. An endpoint or module that takes an email and returns: {valid: true|false, reason: string}.
2. A demo page or script proving real-time operation.
3. Unit tests covering success, invalid syntax, unreachable domain, and nonexistent mailbox cases.
4. Clear instructions for any third-party libraries, DNS settings, or ports that need to be opened.

If you have experience with tools like Node-Mailer, SMTP-client libraries, or popular validation APIs, please mention it—speed and reliability are my highest priorities.