The second process is for queueing local submits. This crashdump is produced, if the dir /var/spool/clientmqueue is not empty and cannot be emptied or has items in queue which can't be managed by the daemon anymore, since they are too old.
First kill all your sendmail-processes, then sudo rm -Rf /var/spool/clientmqueue and finally start sendmail again with /System/Library/StartupItems/Sendmail/Sendmail start hopefully the crashdump will disappear. Also delete sendmail.crash.log since it may have grown to a large crash file. ;-)
Michael C. Haller