java: how can i verify an email address to be valid without sending a confirmation e-mail?

No. There is no way to tell if an email address points to a valid destination. You can check basic syntax, and that the domain has a record in DNS, but that's all.

You can at highest use regex to check if the address is syntactically valid and/or looking up the MX records if the domain is valid, but this still does not guarantee that the email address is legitimate and belongs to the registrant in question. There is really no more reliable way than sending a verification email and waiting for confirmation in a short time-span.

contrary to some of the uneducated answers you can TRY and connect to an MX server and use the VRFY command to see if the sever supports it. Here is a website that will do this for you as an example. If you look at its exchange with the MX server it actually does try and send an email but does a RESET before actually sending it. Testing it with my email address works but I don't get an email. So yes you CAN do what everyone else is saying you can't do. Use an address you know works for your domain, then use one that you don't isn't supported. You will get a 550 on the last RCTP TO command. That is how you know it doesn't exist.