What type of mail is described as being rectangular, 5 inches long, 3.5 inches high, and .007 inches thick?

The correct answer is a letter. In the context of mail classification, a letter is defined as a rectangular piece of mail that adheres to specific size requirements. For a piece to be classified as a letter, it typically needs to be within a particular dimension range, which includes a length of 5 inches and a height of 3.5 inches.

When considering the thickness, letters generally need to be thin, and a thickness of 0.007 inches falls well within those acceptable measurements for standard letter mail. Letters are often used for personal or business communications and have different postage requirements compared to other mail types.

Although parcels, flats, and postcards are also common mail types, they do not conform to the same size specifications as a letter. Parcels are larger and thicker, flats are typically broader but can be longer and thicker than what is described, and postcards are usually a standardized size that differs from the specific measurements mentioned. Thus, the classified dimensions clearly align with those typical of a letter.