28 CFR 540.17 - Correspondence between confined inmates.
An inmate may be permitted to correspond with an inmate confined in any other penal or correctional institution if the other inmate is either a member of the immediate family, or is a party or witness in a legal action in which both inmates are involved. Such correspondence may be approved in other exceptional circumstances, with particular regard to the security level of the institution, the nature of the relationship between the two inmates, and whether the inmate has other regular correspondence. The following additional limitations apply:
(1) The appropriate unit manager at each institution must approve of the correspondence if both inmates are housed in Federal institutions and both inmates are members of the same immediate family or are a party or witness in a legal action in which both inmates are involved.
(2) The Wardens of both institutions must approve of the correspondence if one of the inmates is housed at a non-Federal institution or if approval is being granted on the basis of exceptional circumstances.