Tweaks and tips configuring the imapproxy package.
Failed to load CA data
/var/log/mail.err in.imapproxyd[<pid>]: main(): Failed to load CA data. Exiting.
You are connecting to an imap server that is not advertising LOGIN so imapproxy is attempting to STARTTLS but you probably didn't tell it where any CertificateAuthority certificates are.
Install ca-certificates and change /etc/imapproxy.conf to say: