43 CFR §2.27

1. (a)Except as outlined in § 2.29 of this subpart, a bureau must exercise due diligence to promptly notify a submitter in writing when it receives a FOIA request if:
   1. (1)The requested information has been designated by the submitter as confidential information under § 2.26(a) of this subpart; or
   2. (2)The requested information has not been designated as confidential information by the submitter under § 2.26(a) of this subpart, but the bureau identifies it as possibly confidential information.
2. (b)If a voluminous number of submitters are involved, the bureau may publish a notice in a manner reasonably calculated to reach the attention of the submitters (for example, in newspapers or newsletters, the bureau's Web site, or the Federal Register) instead of providing a written notice to each submitter.