There have been a few incidents of mail purporting to contain the chemical poison ricin.

Ricin is made from castor beans, a plant that is plentiful in many areas of the world, including the United States. Castor beans are used to make castor oil and other beneficial products used for many purposes. In fact, castor oil is often used in the manufacture of paper, including paper used as envelopes. Trace amounts of castor are present in many common items. The process for making ricin from castor beans is rather difficult and quite dangerous. To cause harm, ricin must be injected, inhaled, or ingested.

How to Limit Exposure to a Suspicious Substance in the Mail?

You can find more guidance on suspected chemical, biological, or radiological contamination from the Centers for Disease Control at cdc.gov