Mailverify does it all for you:

An accurate syntax check
A domain check by looking up MX records
A username test using SMTP,EXPN and VRFY

We've packed a full set of solid, professional networking code into the application

MailVerify works by performing a special type of DNS lookup . This lookup ensures that the domain of the email address is fully qualified to receive email. Although it is impossible for validation to be 100% foolproof, MailVerify can reduce the bad email addresses in your database by up to 70% or more! improvings the quality of your email addresses by filtering out more bad addresses than any email validation tool.