What is an essential part of addressing an envelope correctly?

Including the ZIP+4 code is critical for ensuring that mail is delivered accurately and efficiently. The ZIP+4 code provides detailed information about the specific delivery route, which helps postal workers identify the recipient's address more precisely. This extended postal code can reduce the likelihood of delays or misdelivered mail, as it specifies not only the area but also the specific components like the building or apartment number when applicable.

While a return address is certainly important for addressing an envelope, it mainly serves the purpose of showing where the mail should be returned if delivery fails. Choosing the correct mail class affects the cost and speed of delivery rather than the correctness of how the address is formatted. Properly sealing an envelope is crucial for ensuring that the contents do not fall out during transit, but it does not contribute to the address's accuracy on the envelope itself. Thus, while all the options relate to mailing successfully, the inclusion of the ZIP+4 code is essential for correct address designation.