What constitutes a Firm Holdout in postal services?

A Firm Holdout in postal services is specifically designed to allow customers to pick up their street addressed mail at a designated facility rather than having it delivered to their home or business. This service caters to individuals or businesses that prefer or require this arrangement for various reasons, such as increased security or convenience.

The other options refer to different types of postal services that do not align with the definition of a Firm Holdout. For instance, a service for all residential customers suggests a broad coverage rather than the specific nature of a Firm Holdout. Similarly, the service mentioned for customers receiving less than 50 letters daily pertains to mail volume and doesn't capture the essence of the Firm Holdout concept. Lastly, delivering mail to secondary addresses does not relate to the pickup mechanism that defines the Firm Holdout service. Therefore, the correct identification of a Firm Holdout is specifically linked to the ability for customers to actively pick up their street addressed mail, confirming option C as accurate.