Go to: http://yourURL.com/wp-admin/admin.php?page=sp-help. On the help tab there are two sections. The first is called Debug Information and the second is called support info.
If you are SendPress Pro user you can copy and paste both sections into a new support ticket to get assistance.
If you are using the free version, start a new topic in the WordPress forums and someone will do their best to help you figure out the problem.
Please keep in mind when submitting this information to the WordPress forums that there could be sensitive information about your site or your server and you may want to remove references to your domain name if you don’t want that information to be public.