(a) Social Security account numbers shall not be visible on the outside of any package sent by mail.

(b) A document sent by mail may only include the Social Security account number of an individual if it is determined by the head of the agency that the inclusion of a Social Security account number is necessary.

(c) The inclusion of a Social Security account number of an individual on a document sent by mail is necessary when—

(1) Required by law; or

(2) Necessary to identify a specific individual and no adequate substitute is available.

(d) Social Security account numbers must be partially redacted in documents sent by mail whenever feasible.