Sender Verification Background
There are many different types of sender verification methods to cover. For the purposes of this book, we cover the sender compute and challenge/response system types.
In a native sender compute setup, the recipient of the email message requires the sender to calculate the result of a certain non-trivial algorithm. The result of the computation is placed into the message headers so that the recipient can know the sender is legitimate and so that the email server will allow the message to be passed. Sender compute systems contain whitelists of addresses the recipient has already communicated with, making the process for previous correspondents less burdensome.
The issue with this approach is that ...