A more thorough explanation:

Definition: Nonmailable is an adjective used to describe a letter or package that cannot be sent through the mail due to its size, contents, or inappropriate labeling.

Examples:

• A package that exceeds the maximum weight limit for mailing is nonmailable.
• A letter containing hazardous materials is nonmailable.
• A package with an obscene label is nonmailable.

The examples illustrate that nonmailable items are those that cannot be sent through the mail due to specific reasons. These reasons may include exceeding weight limits, containing hazardous materials, or having inappropriate labeling. In each case, the item cannot be transported through the mail and must be sent through alternative means.