ssl_dh

https://wiki.dovecot.org/SSL/DovecotConfiguration says:

"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.