Pro PHP Security: From Application Security Principles to the Implementation of XSS Defenses, Second Edition

Using a Working Email Address for Identity Verification

Many online applications demand possession of a valid email address as a condition of membership, imagining it to be a proof of identity. But it is trivially easy to make up a valid email address, and having a valid email address should never be confused with having a working email address. A user with an actual working email address is thought to be findable.

Even though the number of email addresses is infinite, the number of domain names is finite, and domains are registered to identifiable entities. The name and address of a mailbox provider, an Internet Service Provider (ISP), or an organization can be determined simply by looking at domain registration records. Since most ISPs are ...