28 CFR § 540.203 - Written correspondence limitations.
(a) General correspondence. General written correspondence as defined by this part, may be limited to six pieces of paper (not larger than 8.5 x 11 inches), double-sided writing permitted, once per calendar week, to and from a single recipient at the discretion of the Warden, except as stated in (c) below. This correspondence is subject to staff inspection for contraband and for content.
(b) Special mail.
(2) All such correspondence is subject to staff inspection in the inmate's presence for contraband and to ensure its qualification as privileged communication with the inmate's attorney. Inmates may not seal such outgoing mail before giving it to staff for processing. After inspection for contraband, the inmate must then seal the approved outgoing mail material in the presence of staff and immediately give the sealed material to the observing staff for further processing.
(c) Frequency and volume limitations. Unless the quantity to be processed becomes unreasonable or the inmate abuses or violates these regulations, there is no frequency or volume limitation on written correspondence with the following entities:
(1) U.S. courts;
(2) Federal judges;
(3) U.S. Attorney's Offices;
(4) Members of U.S. Congress;
(5) The Bureau of Prisons;
(6) Other federal law enforcement entities; or
(7) The inmate's attorney (privileged communications only).