What is the maximum thickness for a machinable letter to be mailed by the USPS?

The maximum thickness for a machinable letter to be mailed by the USPS is indeed 0.25 inches. This specification is important for ensuring that the letter can be processed efficiently through USPS machinery. Letters that exceed this thickness may not fit properly into the sorting machines, potentially leading to delays or being categorized as non-machinable, which could result in higher postage rates or different handling procedures. Understanding these dimensions is critical for anyone preparing mail for delivery, as it helps ensure compliance with postal regulations and helps in proper categorization for shipping rates.