28 C.F.R. § 540.12
Controls and procedures
(a) The Warden shall establish and exercise controls to protect individuals, and the security, discipline, and good order of the institution. The size, complexity, and security level of the institution, the degree of sophistication of the inmates confined, and other variables require flexibility in correspondence procedures. All Wardens shall establish open general correspondence procedures.
(b) Staff shall inform each inmate in writing promptly after arrival at an institution of that institution's rules for handling of inmate mail. This notice includes the following statement:
The staff of each institution of the Bureau of Prisons has the authority to open all mail addressed to you before it is delivered to you. “Special Mail” (mail from the President and Vice President of the U.S., attorneys, Members of the U.S. Congress, Embassies and Consulates, the U.S. Department of Justice (excluding the Bureau of Prisons but including U.S. Attorneys), other Federal law enforcement officers, State Attorneys General, Prosecuting Attorneys, Governors, U.S. Courts (including U.S. Probation Officers), and State Courts) may be opened only in your presence to be checked for contraband. This procedure occurs only if the sender is adequately identified on the envelope and the front of the envelope is marked “Special Mail—Open only in the presence of the inmate.” Other mail may be opened and read by the staff.
If you do not want your general correspondence opened and read, the Bureau will return it to the Postal Service. This means that you will not receive such mail. You may choose whether you want your general correspondence delivered to you subject to the above conditions, or returned to the Postal Service. Whatever your choice, special mail will be delivered to you, after it is opened in your presence and checked for contraband. You can make your choice by signing part I or part II.
I have read or had read to me the foregoing notice regarding mail. I do not want my general correspondence opened and read. I REQUEST THAT THE BUREAU OF PRISONS RETURN MY GENERAL CORRESPONDENCE TO THE POSTAL SERVICE. I understand that special mail will be delivered to me, after it is opened in my presence and checked for contraband.
I have read or had read to me the foregoing notice regarding mail, I WISH TO RECEIVE MY GENERAL CORRESPONDENCE. I understand that the Bureau of Prisons may open and read my general correspondence if I choose to receive same. I also understand that special mail will be delivered to me, after it is opened in my presence and checked for contraband.
Inmate (Name), (Reg. No.), refused to sign this form. He (she) was advised by me that the Bureau of Prisons retains the authority to open and read all general correspondence. The inmate was also advised that his (her) refusal to sign this form will be interpreted as an indication that he (she) wishes to receive general correspondence subject to the conditions in part II above.
(c) Staff shall inform an inmate that letters placed in the U.S. Mail are placed there at the request of the inmate and the inmate must assume responsibility for the contents of each letter. Correspondence containing threats, extortion, etc., may result in prosecution for violation of federal laws. When such material is discovered, the inmate may be subject to disciplinary action, the written material may be copied, and all material may be referred to the appropriate law enforcement agency for prosecution.
(d) The inmate is responsible for filling out the return address completely on envelopes provided for the inmate's use by the institution. If the inmate uses an envelope not provided by the institution, the inmate is responsible for ensuring that the envelope used contains all return address information listed on the envelope provided by the institution.