I am trying to implement a feature in the email client that we are developing to find the list of email addresses in a particular domain, once the end user keys in the first 3 letters in to/cc/bcc box. Since the mails will be sent only to one predefined domain (which is internal).
In general, there's no way to find all the valid email addresses in a domain. And even in cases where it is possible, there's no standard way to do it. JavaMail isn't going to help you here. If your mail server supports such a capability, you're going to need to figure out how to do it in a product-specific way.