Type /etc/init.d/sendmail stop and press Return.
This kills the old sendmail daemon to keep it from trying to process the old queue directory.
Type cd /var/spool and press Return.
Type mv mqueue omqueue; mkdir mqueue and press Return.
This moves the directory, mqueue, and all its contents to the omqueue directory and then creates a new empty Rmqueue directory.
Type chmod 755 mqueue; chown daemon.daemon mqueue and press Return.
These commands set the permissions of the directory to read/write/execute by owner, and read/execute by group and others; these commands also set the owner and group to daemon.
Type /etc/init.d/sendmail start and press Return.
This starts a new sendmail daemon.