I had some problem where my sendmail server was not sending mail. It wasn't sending mail for four days. I tried restarting sendmail. That didn't help. I stopped sendmail and deleted everything in /var/spool/mqueue and /var/spool/mqueue-client (also /var/spool/clientmqueue). When I restarted sendmail the problem went away. I never did figure out which message was causing the problem. In the logs it was complaining about trying to send mail to localhost 127.0.0.1. I don't know what was going on.