32 CFR § 536.67 - Mailing procedures.
Thirty or sixty day letters seeking information from claimants, final offers and denial notices are time-sensitive as they require a claimant to take additional action within certain time limits. Accordingly, follow procedures to ensure that the date of mailing and receipt of a request for reconsideration are documented. Use certified mail with return receipt requested (or registered mail, if being sent to a foreign country other than by the military postal system) to mail such notices. Upon receipt, an appeal or request for reconsideration will be date-time stamped, logged in, and acknowledged as set forth in § 536.68.
See also AR 27-20, paragraph 13-5, and DA Pam 27-162, paragraph 2-77.