This script works as follows.
It takes the email address, and finds the domain part. It then feeds
it to dig.r ( using 'get-mx-ip), which is able to determine the mail
server responsible for that domain by looking up the mx records at
your local dns server.
The smtp scheme uses system/schemes/default/host as the mail server,
and this is then set to the value obtained above. The script uses
'set-net, but you can set it directly instead.
The script then sends what is known as a smtp challenge to that mail
server. A smtp challenge uses the null from address of "<>",
and then sends a "RCPT TO" command to that server. If the server
recognises that adress, it answers appropriately. If not, an error
is generated by the smtp scheme, and we know that that email address
does not exist. If it exists, or not, we then issue a "QUIT" command
by simply closing the smtp connection.
Note that some email servers will respond okay to every query to
stop email harvesting.
The smtp scheme is used instead of the esmtp scheme as the latter
is not necessary for this, and furthermore has smarts in it which
prevent it being used in this way.