Are the letters encrypted?

Yes, your data, including your letters, is transmitted securely on Slowly via TLS 1.2/1.3 to an HTTPS endpoint. This means that all data, both while in transit and when stored, is encrypted. The encryption is provided by Cloudflare and Amazon Web Services (AWS). 

While this encryption ensures the security of your data, it's important to remember not to share any personal or sensitive information through Slowly or any other messaging platform for your own privacy and security.