Beginning with V8.7, sendmail began to support a flat text-file form of database. The /etc/hosts file is an example of such a flat file, in that it is organized in a line-by-line manner:

123.45.67.89      here.our.domain

When such files are read as databases (with the text type, text), you need to specify which column contains the key and which contains the value.

For nisplus, netinfo, and ph database maps, the -k switch specifies the name (text) of the desired column.

When the -k switch specifies which column contains the key, its absence defaults to 0 for the text type (which is indexed beginning with 0), and defaults to the name of the first column for the nisplus type. See also -v (-v) for the returned value's column, and -z (-z) for the column delimiter.

Finally, note that for ldap database maps the -k switch has a different meaning, one that is particular to that type.