If you just want to verify that a string is in proper email format, use regular expressions. For an example, see here:
http://msdn.microsoft.com/en-us/library/01escwtf.aspx[
^]
If you want to verify whether the address is valid and active, there's no perfect way to do this. You could send a test email from a specific address, and then check new mail for the from-address that you used, and if you get a bounce email, parse the body to figure out if the bounce is for the email you sent. This assumes that the target host is configured to send back bounce messages.
At a simpler level, you could resolve the MX record for the domain and see if exists. That at least guarantees a valid host. Of course this won't verify the username for you.
And then there are email servers that are configured to swallow emails to non-existent users (this is an anti-spam method to prevent spammers from detecting bad addresses or real ones). Not much you can do about those really.