A letter was a type of written correspondence.
The First Doctor left the Monk a letter explaining he had disabled the Monk's TARDIS. (TV: The Time Meddler)
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)
Reinette Poison wrote the Tenth Doctor a letter musing on when he would return to her, hoping he would return before she succumbed to her illness. The Doctor eventually recieved the letter after Reinette's passing. He read it aboard his his TARDIS. (TV: The Girl in the Fireplace)
Umbreen's second husband wrote her letters while he was away. She gave these letters to their daughter Najia on Umbreen's birthday in 2018. (TV: Demons of the Punjab)
Ryan Sinclair's father, Aaron, (TV: Resolution) sent him a letter apologising for failing to attend the funeral of his own mother, Grace O'Brien. He offered Ryan a place to live with him as they were "proper family" unlike Grace's second husband, Graham. (TV: Arachnids in the UK)
