After switching KEY_ALGO to secp384r1 my email-clients weren’t able to connect to dovecot anymore.

This was what I found in the log.

TLS handshaking: SSL_accept() failed: error:1408A0C1:SSL routines:ssl3_get_client_hello:no shared cipher,

after switching KEY_ALGO back to rsa anything works as before.

It will still need some more time, till the new security-level will arrive at the end-users.