Email Validation (Real-time, Bulk)

What is email validation? Why is email validation important? What's the difference between real-time and bulk validation? When should I use real-time validation (e.g., API)? When should I use bulk validation? How accurate are email validation services? What types of checks do validation services perform (syntax, MX, SMTP)? How do syntax checks differ from mailbox checks? What is SMTP handshake validation? What are catch-all domains and why are they tricky? What is “accept-all” behavior and how should it be labeled? What is MX validation? What is domain existence verification? How do throttled SMTP responses affect validation accuracy? Why are temporary failures different from hard bounces? Why does validation not guarantee deliverability? What are typical validation accuracy benchmarks? How to test or audit a validation vendor? How to spot fake “validation” services that only check syntax?