Keep in mind that DKIM relies on patented technology that may require a separate licence.
As far as software copyright is concerned, zdkimfilter is free software: you can redistribute it and/or modify it under the terms of the GNU General Public Licence as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
zdkimfilter is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public Licence for more details.
As an additional permission under GNU GPLv3 section 7,
If you modify zdkimfilter, or any covered part of it, by linking or combining it with OpenSSL, OpenDKIM, Sendmail, or any software developed by The Trusted Domain Project or Sendmail Inc., containing parts covered by the applicable licence, the licensor of zdkimfilter grants you additional permission to convey the resulting work.