Which of the following is NOT a type of mail delivery service?

The correct choice highlights that "Random delivery" is not a recognized or standardized type of mail delivery service. In contrast, Priority mail, Express mail, and Courier service are all legitimate and established methods of delivering mail, each with specific features and purposes.

Priority mail is typically used for faster delivery of letters and packages within the postal system, offering a dependable service with a set delivery timeframe. Express mail, on the other hand, is even faster and guarantees next-day delivery for urgent items. Courier services involve private companies that specialize in swift and secure delivery, often providing same-day or next-day service tailored to businesses and individuals requiring quick turnaround times.

In this context, since "Random delivery" does not align with any recognized mail delivery methods and lacks the structured framework and operational principles of the other options, it is correctly identified as not being a type of mail delivery service.