sendmail, 4th Edition

-m

Suppress replacement on match V8.1 and later

Ordinarily, a successful lookup in a database map causes the key to be replaced by its value. When the intention is to merely verify that the key exists (not to replace it) the -m switch can be used to suppress replacement.

For example, the values that are returned from the hosts.byname NIS database map are not generally useful (they contain multiple hostnames). In looking up a key in this database map (with $( and $); see Use $( and $) in Rules on page 892), the -m switch prevents those multiple names from wrongly replacing the single hostname in the key. Note that the -a switch (-a on page 887) can still be used to append a suffix to a successful lookup. Also, the $:default (Specify a Default with $: on page 893) is still used if the lookup fails.