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... (Budget: $30 - $250 USD, Jobs: API Development, C# Programming, JavaScript, MySQL, PHP, RESTful API, SMTP, Software Architecture)