Which dimension is not acceptable for an automation compatible letter-sized mailpiece?

The dimension of 11 ½ inches in height is not acceptable for an automation compatible letter-sized mailpiece. For automation, the United States Postal Service has specific size requirements that must be adhered to in order to qualify for automated processing. A letter-sized mailpiece must be within a defined range of dimensions, specifically between 3 ½ inches to 6 1/8 inches in height, and a maximum height of 11 ½ inches is only acceptable for larger mail categories, specifically flat mail.

In this context, the incorrect heights of the other options align with the requirements for automation compatibility. Both 5 ½ inches, 7 inches, and 8 inches fit within the acceptable height range for automation letter-sized mailpieces, while 11 ½ inches exceeds the permissible height for letters, positioning it outside of the criteria set for automated processing. Thus, recognizing the dimensional restrictions of the automation specifications is crucial in understanding why 11 ½ inches does not qualify.