mkdhparams — Create new DH parameters for the Courier Mail Server
/usr/lib/courier/sbin/mkdhparams 
      This script is intended to be executed from a cron job, monthly.
      If the file /etc/courier/dhparams.pem is older than
      25 days, the script creates a new file. This file contains pregenerated
      DH parameters for the Courier Mail Server.
    
      The DH_BITS environment variable, if set controls the
      length of the generated DH parameters. It gets passed, verbatim, to
      either the dhparam OpenSSL command, or the
      --sec-param option to GnuTLS's
      certtool. The default value is 768 for OpenSSL,
      and “high” for GnuTLS.