Yes, mail is considered sacred. All mail is delivered to SHU inmates Monday through Friday and they are allowed to write you as much as they want. The limitations are on telephone use (one 15-minute call per week), commissary and visitation.
https://www.inmateaid.com/ask-the-inmate/are-inmates-allowed-to-send-and-receive-mail-while-in-the-shu#answer
Accepted Answer Date Created: March 14,2016