Bernie Madoff has email in prison. Huh?

Today’s new york times has an article about Bernie Madoff. The journalist writes that much of his information comes from email messages that he and Madoff sent each other.

I was surprised that prisoners have access to email at all. Is this common, and how does it work?

The journalist does mention that the emails were “monitored” by prison authorities, apparently just like all regular mail or packages sent to/from prisoners.
But surely email is not like regular mail? A computer with internet access seems wide open to abuse. It would give a prisoner access to his victims, and to his fellow cohorts planning more crimes, etc.
Even if the prisoner is not hooked directly to the internet*, isn’t it possible that he -or his friends on the outside- could still embed secret messages, with viruses or whatever.

So how does this work in the jail setting?
*(I’m guesssing: he is allowed to type his message on a separate computer, copy it to a disc and then a prison guard copies the disc to a different computer and types in the email address for sending)

Here is a website for Trust Fund Limited Inmate Computer System (TRULINCS), “a new program currently being deployed by the Federal Bureau of Prisons (BOP) to provide inmates with some limited computer access, to include the capability to send and receive electronic messages without having access to the Internet.”