High Quality Content by WIKIPEDIA articles! A mail sack or mailsack as defined by the National Postal Museum is a United States Postal Service lower security class mail bag used to carry second-class mail as well as third-class mail and fourth-class mail. The use of a particular kind or designation of "mail sack" is dependent upon the type of mail it contains, and the security it requires. Mail sacks of the United Kingdom come in different colors for sorting different kinds of mail, usually transparent to easily identify the different types of mail.