The answer is difficult to give because the restrictions placed on an inmate for this sort of crime are not often made public. Domestic terrorism defendants are oftentimes placed in segregation for security reasons making their plight even less known as other inmates have no access to them either.
From what we know for sure, the answer is "yes" they can receive and send mail... BUT, the process is highly monitored and censored where deemed appropriate. If the decision makers have any trepidation that the communication is inappropriate or could be a security risk the letters will not go through. If you are a friend and keep the conversation light and not about the crime, punishment, government or process you should be okay.
https://www.inmateaid.com/ask-the-inmate/hello-do-inmates-who-are-accused-or-found-guilty-of-committing-domestic-terrorism-able-to-send-and-receive-mail#answer
Accepted Answer Date Created: November 14,2014