If neither -N nor -O is specified, sendmail uses an adaptive algorithm to decide whether to look for the terminating zero byte. The algorithm starts by accepting either possibility. If the first key looked up is found to end with a terminating zero byte, the algorithm will thereafter look only for keys with a terminating zero byte. If the first key that is looked up is found to not end with a terminating zero byte, the algorithm will thereafter look only for keys without a terminating zero byte.

If this -O switch is specified, sendmail never tries a zero byte, which can speed matches. Note that if both -N and -O are specified, sendmail will not produce an error message, and will never try to match at all, thus causing all lookups to appear to fail.