The short version of you don't want to know all the technical details:
If you choose a ECDSA key for your web server, make sure you have also a RSA key for SMTP inbound connections
In case you are interested in the technical details, read on ...
This is a companion discussion topic for the original entry at https://blog.nashcom.de/nashcomblog.nsf/dx/important-for-domino-smtp-with-ecdsa-keys-for-starttls.htm