A letter was a type of written correspondence.
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)
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)