I'd like to try this or the similar delta.chat, but have thousands of confidential/sensitive e-mails on my account and don't know what these clients will do to/

I'd like to try this or the similar delta.chat, but have thousands of confidential/sensitive e-mails on my account and don't know what these clients will do to/with them. It's also not an ideal situation from a security perspective (a lot of damage could be done if the clients are exploitable).