Mail is considered sacred in the penal system and even inmates in solitary confinement, administrative hold or the SHU are able to receive and write letters in any amount at any time.
https://www.inmateaid.com/ask-the-inmate/are-inmates-that-are-in-solitary-still-able-to-send-and-receive-letters#answer
Accepted Answer Date Created: June 02,2016