“[A] prisoner's right to the free flow of incoming and outgoing mail is protected by the First Amendment.” Davis v. Goord, 320 F.3d 346, 351 (2d Cir. 2003). “[T]interference with prisoners’ mail ‘must be no greater than is necessary or essential to the protection of the particular governmental interest involved. Washington v. James, 782 F.2d 1134, 1139 (2d Cir.1986) (quotation marks omitted). Prison mail regulations are valid only if they are “reasonably related to legitimate penological interest.” Turner v. Safley, 482 U.S. 78, 89 (1987).
“Prison walls do not form a barrier separating prison inmates from the protections of the Constitution,” Turner v. Safley, 482 U.S. at 84. “[N]or do they bar free citizens from exercising their own constitutional rights by reaching out to those on the ‘inside.’” Thornburgh v. Abbott, 490 U.S. 401, 407 (1987). “Access [to prisoners] is essential to ... families and friends of prisoners who seek to sustain relationships” with prisoners. Id.