Checking email address

[meritj @ fincen . treas . gov (Jim Meritt)]

I know how to "check" email addresses (well, a lot of them) by telnetting
to the hostname post 25 and querying the sendmail.

Unfortunately, not even this works if there is a firewall (or two) in the way.
How can I verify email addresses (without trying it)?

Jim Meritt