You can check if it is a VALID email address by checking it against the corresponding RFC. This mainly means there must be an @-sign somewhere in there with something in front and behind. Then you could try a DNS lookup of the part after the @-sign. If that doesn't exist it's pointing to a non-existing domain and would therefore never arrive (not quite sure here, it is possible that for example dummy.somisp.sometld would arrive is only somisp.sometld is a valid DNS entry). The part in front of the @-sign you can only determine the validity of by actually trying to send to the address. If you get a bounce (which may take a long time to arrive, some mailservers try repeatedly for a day or more to deliver in case a client address is only temporarilly unavailable) the address certainly does not exist. If you get no bounce someone has received your message, but that may well be a blacklist and no recipient will ever read it.
Hi, Welcome to JavaRanch! The SMTP standard includes a "VRFY" command which you used to be able to use to verify whether an email address was valid at a particular host. Nowadays, virtually all sites implement this as a no-op, so it's pretty much useless. The only way that works is to send an email to the address containing a unique URL. The email should instruct the recipient to visit the URL. A servlet at the URL can then record that the corresponding email is valid.