Which of the following is NOT a requirement for sending First-Class Mail?

Sending First-Class Mail requires that certain standards be met to ensure proper processing and delivery. While affixing correct postage and using the proper address format are essential requirements, delivery confirmation is an optional service that can be added if desired but is not mandatory.

The correct postage ensures that the mail is adequately paid for its weight and destination, while the proper address format ensures that the mail can be efficiently routed to its recipient. The inclusion of hazardous materials can complicate the mailing process and is generally restricted, ensuring safety during handling and transport. However, opting for delivery confirmation is entirely up to the sender and not a necessary condition for sending First-Class Mail, which clarifies why this option stands out as not a requirement.