Dead Letter Mail (also known as a Dead Letter) is mail that cannot be delivered to the addressee, usually due to lack of compliance with postal regulations. Largely based on the British model that emerged in the late eighteenth century, many countries developed similar systems for processing undeliverable mail. The term Dead Letter is colloqually applied to any set of instructions, especially a law, that has lost its authority without being formally abollished or repealed.

The term 'dead mail' is perhaps a misnomer, and several jurisdictions have opted for the use of the term 'undeliverable mail' as more clearly representing the status of the item whose transmission has been impeded. Being internal departments within postal administrations, little information about the Dead letter office or Undeliverable Mail Office function has ever been made public. A few journal articles and at least one recently published book (Canadian) dealing with this topic have appeared.

It is anticipated that with increased collection and study of the postal markings originating from such offices, the body of literature on this obscure topic will grow.