There are no restrictions on incoming postal mail
https://www.inmateaid.com/ask-the-inmate/how-can-i-write-an-inmate-if-i-am-not-yet-on-the-approved-email-list-and-he-doesn-39-t-know-i-39-m-trying-to-write-him#answer
Accepted Answer Date Created: September 20,2019