Hello, I landed here by googling "httpd syno_mail.c Failed to send mail" and found that MadJik has the same problem as I do with a synology server. After asking their tech support I solved the problem and thought it might help other people who google the same thing so i post their answer here.
You need to go to the "control center" ==> "notification configuration" ==> "e-mail" tab to set up the system mail notification. (I am using a Chinese version so the above texts are just English approximations.) (failed to attach a screenshot.)
