Lettergram is an AI-powered service that sends personalised letters to your physical mailbox. Users can select a pen pal from various categories, including historic figures, child pen pals, adult pen pals, and fantasy pen pals. Each pen pal has its own unique backstory and persona, and users can respond to their letters by writing a physical letter and sending it back in a return envelope. The service aims to bring back the nostalgia of receiving letters in the mail and provide an immersive experience for users.