The -N switch tells makemap to include a trailing zero byte with each key that it adds to the database. When sendmail looks up a key in the database, it uses a binary comparison. Some databases, such as /etc/aliases under SunOS, append a zero byte to each key. When a trailing zero byte is included with a key, it must also be included with the tokens being looked up, or the lookup will fail. The use of this switch must match the K command (-N).