To generate a new DH parameters file (this will take very long):
# openssl dhparam -out /etc/dovecot/dh.pem 4096
then add the file to /etc/dovecot/conf.d/10-ssl.conf
ssl_dh = </etc/dovecot/dh.pem
This creates a file dh.pem. The dh.pem file contains the base g and modulo p. These parameters define how OpenSSL performs the Diffie-Hellman (DH) key-exchange.
More light reading:
Yes it took a very long time, indeed five hours in my case. But now it works.
I took a nap and listened to Messiah while it ground away...
:-)