EmailVerify.NET
EmailVerify.NET is a powerful .NET component which verifies email addresses using various techniques, including: syntax verification (according to RFC 2821 and RFC 2822), MX record lookup and disposable address (DEA) validation, SMTP availability check, mailbox existence check and catch-all address verification. It is completely written in managed code (C#) and is compliant with the Common Language Specification (CLS), so it can be used with any other .NET language.