sendmail, 4th Edition

-f

Preserve case V8.1 and later

Ordinarily, sendmail will normalize a key to lowercase before looking it up in a database. If the keys in the database are case-sensitive (“TEX” is considered different from “tex,” for example), the -f database switch should be used to prevent this normalization. Note that if the -f switch is omitted (the default), the database must have been created with all lowercase keys (also the default).

Also note that when the -f switch is used with the regex database-map type, it causes the regular expression match to be made in a case-insensitive manner.