Hazardous mailing is a term used to describe the process of sending items that may contain hazardous materials, such as anthrax. Anthrax is a potentially fatal disease caused by the bacterium Bacillus anthracis. It can be spread through contact with infected animals or contaminated materials, such as mail. As such, it is important to take precautions when sending mail that may contain hazardous materials, such as anthrax.