Letter writing in Indian history

Can anybody tell me of any examples of letter-writing in Indian history or mythology?

Wasn't there a story about a queen in Rajasthan who wrote to a king asking him for his protection? Google reveals that Maharani Karnawati sent a rakhi to Emperor Humayun, but a rakhi's not the same as a letter, so I'm not sure if it's the same episode or something different...

If you can shed some light on this or have any other ideas, let me know.