What is the best way to validate that e-mail addresses are deliverable? I'm not talking about checking the formulation of the address (name@somedomain.com) but rather getting a response back from a mail server that the address is valid.

I could process the delivery confirmation messages by sending an actual message, but I'd prefer NOT to send an actual message to the addressee, if possible.

Anyone know if (and more importantly how) this is possible?

TIA