28 CFR § 540.14 - General correspondence.
(a) Institution staff shall open and inspect all incoming general correspondence. Incoming general correspondence may be read as frequently as deemed necessary to maintain security or monitor a particular problem confronting an inmate.
(b) Except for “special mail,” outgoing mail from a pretrial inmate may not be sealed by the inmate and may be read and inspected by staff.
(c)
(1) Outgoing mail from a sentenced inmate in a minimum or low security level institution may be sealed by the inmate and, except as provided for in paragraphs (c)(1)(i) through (iv) of this section, is sent out unopened and uninspected. Staff may open a sentenced inmate's outgoing general correspondence:
(i) If there is reason to believe it would interfere with the orderly running of the institution, that it would be threatening to the recipient, or that it would facilitate criminal activity;
(ii) If the inmate is on a restricted correspondence list;
(iii) If the correspondence is between inmates (see § 540.17); or
(iv) If the envelope has an incomplete return address.
(2) Except for “special mail,” outgoing mail from a sentenced inmate in a medium or high security level institution, or an administrative institution may not be sealed by the inmate and may be read and inspected by staff.
(d) The Warden may reject correspondence sent by or to an imate if it is determined detrimental to the security, good order, or discipline of the institution, to the protection of the public, or if it might facilitate criminal activity. Correspondence which may be rejected by a Warden includes, but is not limited to, correspondence which contains any of the following:
(1) Matter which is nonmailable under law or postal regulations;
(2) Matter which depicts, describes, or encourages activities which may lead to the use of physical violence or group disruption;
(3) Information of escape plots, of plans to commit illegal activities, or to violate Bureau rules or institution guidelines;
(4) Direction of an inmate's business (See § 541.13, Prohibited Act No. 408). An inmate, unless a pre-trial detainee, may not direct a business while confined.
(5) Threats, extortion, obscenity, or gratuitous profanity;
(6) A code;
(7) Sexually explicit material (for example, personal photographs) which by its nature or content poses a threat to an individual's personal safety or security, or to institution good order; or
(8) Contraband. (See § 500.1 of this chapter. A package received without prior authorization by the Warden is considered to be contraband.)