Double Opt-In Email
WebDynamic™ Web Glossary
With double opt-in, a submitted email address is not immediately added to a mailing list. Instead, an email is sent to the address, asking to confirm that the name should indeed be added. If the recipient of the confirmation email does nothing, the submitted address is taken off of any mailings. The name is only added to a distribution list if the recipient responds to the confirmation email.
This is done to verify genuine email addresses from the spam addresses.