Ordinarily, in the case of types that employ disk files, sendmail will complain if a specified file cannot be opened for reading. If the presence of a database file is optional (as it can be on certain machines), the -o switch should be used to tell sendmail that the database is optional. Note that if a database is optional and cannot be opened, all lookups will silently fail for rules that use that database.

Also note that for network-based types of database maps, this -o switch can be used to cause failed initializations to be ignored. If a database map is used during the processing of a message, and if a lookup fails in the absence of a -o switch, the message (or SMTP request) will be rejected with a temporary failure.