From: "Jim Washington" <jwashin@vt.edu>
Apparently _some_ of this is possible. A couple of weeks ago, I was facing a similar situation, and found a site on the web that would do validation of email addresses by (IIRC) sending a mailto header to the mail domain and looking at the response. It did not work for all domains, notably hotmail and yahoo would not tell whether the address was OK or not. But a few smaller isps would work. Unfortunately, I cannot find that site now.
Yes, you can look up the domain, find the mailhandler, open a connection, do a MAIL FROM and a RCPT TO to the address in question. If that goes OK, the mail shouldn't bounce. There is a dns module at http://www.interlink.com.au/anthony/ , and then it should not be too hard to make something that verifies e-mail addresses. That might be rather handy... But I won't need one for a couple of months so I'm not doing it. :-)