Letter

A letter was a type of written correspondence.

Sam thought he'd contracted anthrax from letters he'd been reading. The First Doctor assured him, though, that it was not anthrax he'd been infected with, but a meme. (PROSE: INtRUsioNs)

Samantha Jones' parents began receiving correspondence from her in 1997, including a letter from 1894 London. (PROSE: Unnatural History)

Vastra sent a letter to Strax requesting his presence in an unconscious "conference call. (TV: The Name of the Doctor)