Fourth-class mail, also known as bulk mail or standard mail, refers to non-urgent and promotional materials sent in bulk. It includes items such as magazines, catalogs, flyers, and advertising circulars. Synonyms for fourth-class mail may include direct mail, advertisement mail, junk mail, and unaddressed advertising mail. These terms reflect the variety of materials that are sent as fourth-class mail. Despite its negative connotations, fourth-class mail plays an important role in the advertising industry, offering a cost-effective way to reach a wide audience. While it may not always be welcomed by recipients, fourth-class mail remains a powerful marketing channel for businesses and organizations.