I have setup DKIM on a Ubuntu 14.04 machine using opendkim and postfix. I generated the RSA keypair and updated the TXT record. When testing the settings at any DKIM tester like www.mail-tester.com or www.dkimvalidator.com, I get errors like “result = fail, bad RSA signature”. Here is an example mail (info from dkimvalidator.com). I substituted the real domain with mail.example.com.