Function try_decrypt 

pub fn try_decrypt<'a>(
    mail: &'a ParsedMail<'a>,
    private_keyring: &'a [SignedSecretKey],
    shared_secrets: &[String],
) -> Result<Option<Message<'static>>>

Tries to decrypt a message, but only if it is structured as an Autocrypt message.

If successful and the message was encrypted, returns the decrypted and decompressed message.