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)
After learning to both read (PROSE: The Dark Path) and write, (PROSE: The Lost) Jamie McCrimmon wrote a letter for Victoria Waterfield, thanking her for her reading and writing lessons, after she departed his and the Second Doctor's company. The Seventh Doctor delivered the letter to an elderly Victoria. (AUDIO: The Story of Extinction)