Experian Data Quality offers an email verification solution that identifies whether an email address is valid and deliverable, so that you know whether an email address is accurate and actionable before mailing to it, leading to improved email marketing.
The solution works by performing multiple checks on the syntax, domain, mail server and account. All validation occurs in real time, meaning no database lookups. This process can be done at the point of collection or in bulk on an ad hoc basis through a hosted Web service that supports REST.
Sample code available is for PHP, C# and JSP/Java.