We noticed the SSL documentation for ZeroSSL only applies to for HTTPS and not for SMTP. We need to apply the SSL certificate also to the SMTP separately. Ideally this either needs to be documented or applied automatically via a script.
Furthurmore, Cloudflare SSL is not an option if you need SSL for SMTP API. This is important if you want to send transactional emails through the SMTP Relay API with TLS and port 587.