Literal "Letter" in the Title:
* The Letter (1940): A classic film noir about a woman accused of murder.
* Letters from Iwo Jima (2006): A powerful war film told from the perspective of Japanese soldiers.
* The Letter Writer (1944): A suspenseful film about a woman who becomes entangled with a dangerous man.
* The Letter Never Sent (1940): A film about a young woman's journey through war-torn Europe.
* The Letter to Elise (1960): A romance film about a woman who finds a letter from her deceased husband.
* The Letter (2018): A Malaysian drama about a woman who receives a mysterious letter.
* A Letter to Momo (2011): An animated film about a young girl who receives a message from her late father.
Titles with a "Letter" Theme:
* Dear John (2010): A romantic drama about a soldier's letters to his girlfriend.
* A Letter to My Killer (2015): A thriller about a woman who receives a letter from her killer.
* Letters to Juliet (2010): A romantic comedy about a woman who finds love after discovering a letter to Juliet.
* The Secret Letter (2018): A film about a family who receives a letter containing a secret that could change their lives.
Bonus:
* The Post (2017): A drama about the publication of the Pentagon Papers, though it's more about the newspaper itself than a single letter.
I hope this list provides some inspiration!