You can use the DnsContextFactory to perform MX lookups. If you search for DnsContextFactory on java.sun.com you will find some pretty good examples. Start there. In the meantime here is a quick example of getting select records for an IP.
The NetworkInterface and InetAddress classes are usefull also. Look at the methods
Ok here is a quick and dirty validator. Gets the email string, validates it. If ok it returns the email if DNS lookup fails returns "<< email >>". Thus resetting the field. This is an AJAX call on the server so it isn't a real validator per see ala JSF or JSP style.
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