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.