update: remove postmaster_address = root, mail_plugins = and enable:...

  #  sieve = yes

  #}

postmaster_address = root

  # This strips the domain name before delivery, since the default

  # userdb in Debian is /etc/passwd, which doesn't include domain

  # names in the user.  If you're using a different userdb backend

  # that does include domain names, you may wish to remove this.  See

  # https://doc.dovecot.org/2.4.0/howto/lmtp/exim.html and

  # https://doc.dovecot.org/2.4.0/core/summaries/settings.html#auth_username_format

  # auth_username_format = %{user | username}

  # Space separated list of plugins to load (default is global mail_plugins).

  mail_plugins =

  # https://doc.dovecot.org/2.4.1/howto/lmtp/exim.html and

  # https://doc.dovecot.org/2.4.1/core/summaries/settings.html#auth_username_format

  auth_username_format = %{user | username | lower}

}