Challenge–response spam filtering

Challenge–response spam filtering is a method of combating email spam by requiring the sender to take a specific action to prove that they are not a spammer before their message is accepted by the recipient's email server or inbox. This approach involves a two-step process: 1. **Challenge**: When an unknown sender sends an email, the recipient's email system automatically generates a "challenge" message, which is typically a request for the sender to complete a simple task.