Mail means any physical package sent to entities or individuals outside the Department through the United States Postal Service or any other express mail carrier; and
Necessary includes only those circumstances in which a component would be unable to comply, in whole or in part, with a legal, regulatory, or policy requirement if prohibited from mailing the full Social Security account number. Including the full Social Security account number of an individual on a document sent by mail is not "necessary" if a legal, regulatory, or policy requirement could be satisfied by either partially redacting the Social Security account number in accordance with paragraph (d)(3) of this section, or entirely removing the Social Security account number.
28 C.F.R. §16.53