ssl_dh is required from 2.3.0-2.3.2. From 2.3.3 onwards its optional. You can rm the ssl-parameters.dat file to get rid of that warning.
"Since v2.3.3+ Diffie-Hellman parameters have been made optional, and
you are encouraged to disable non-ECC DH algorithms completely."
and a bit later:
"From version 2.3, you must specify path to DH parameters file using
ssl_dh=</path/to/dh.pem"
So.
1. Is ssl_dh an optional or a must?
2. I've disabled ssl_dh in my config. Dovecot works fine except it shows
warnings:
doveconf: Warning: please set ssl_dh=</etc/dovecot/dh.pem
doveconf: Warning: You can generate it with: dd ...
I'm using dovecot version 2.3.4.1-1~bpo9+1 from debian stretch-backports
--
sergio.