I had a few users who have been using my bcSpamBlock WordPress plugin for blocking spam tell me that it also completely blocks trackbacks. Thanks to their input and specifically wlx who sent me a patch to have it skip validation on trackbacks and pingbacks.
I modified the WordPress component of the code so that it doesn’t check for the cryptographic signature for trackbacks, but instead makes sure that the remote host is the webserver for the site that it says it is. If that check passes, it also retrieves the page and verifies that it contains a link to your own blog.
I’ve updated the plugin and it is available for download on the bcSpamBlock wordpress page