What must mail clerks do with undeliverable mail?

When handling undeliverable mail, mail clerks must follow specific protocols to ensure that the mail is dealt with correctly and efficiently. The correct response, which states that undeliverable mail should be returned daily, reflects standard operating procedures that help maintain the integrity of the mailing system.

Returning undeliverable mail daily allows for timely sorting and processing of mail that cannot reach its intended recipient for various reasons, such as incorrect addresses or recipients no longer living at the given address. This method also helps in keeping the postal system organized and avoiding backlogs, ensuring that mail-handling flows remain smooth.

In contrast, holding undeliverable mail until further notice may lead to confusion and delays in the delivery system. Storing undeliverable mail in a safe location may create unnecessary clutter and may not comply with required postal regulations regarding processing times. Lastly, improper disposal of mail can lead to privacy violations, as it may contain sensitive information, and therefore is not an acceptable practice. Thus, returning undeliverable mail daily is the best approach not only for efficiency but also for compliance with postal regulations.