Dead letter mail is correspondence that cannot be delivered to its addressee — historically because of an incomplete or non-compliant address, an undeliverable return address, or because both correspondents moved before the letter caught up to either of them.
National postal systems have long maintained dedicated dead-letter offices to triage such mail: open it to find clues, attempt forwarding, return it to sender when possible, and otherwise destroy or auction its contents under jurisdictionally specific rules. In internet usage, the phrase has migrated outward to describe any message sent to a recipient whose stature, notoriety, or simple inbox volume makes a reply structurally unlikely — a cold outreach to a celebrity, a pitch to a high-volume editor, a cold DM to a category-leading founder.
The reframing turns a postal artifact into a probabilistic concept: a dead letter is not undeliverable, just statistically unlikely to land — which is why the craft of cold outreach is really the work of writing a message that beats those odds.