When receiving this error you are either having a STARTTLS problem or there is a chance that it is a bug in sendmail and you just need to supply a patch to sendmail. First to resolve the issue follow the below steps.
1. Stop sendmail: /etc/init.d/sendmail stop
2. Make sure the PID’s all stopped: ps -ef | grep sendmail
3. If not kill the stuck process ID’s: kill -9
4. Also check the PID’s from the maillog where you originally saw the error and make sure those are not running.
5. Start sendmail: /etc/init.d/sendmail start