Wikipedia
Corrlinks is a privately owned company that operates the Trust Fund Limited Inmate Computer System (TRULINCS), the email system used by the United State Federal Bureau of Prisons to allow inmates to communicate with the outside world. TRULINCS is a fee-based system that inmates must pay for in order to send or receive email. Unlike commercial sites which allow correspondents to send an email which is then printed and mailed to an inmate, this service provides direct email access to federal inmates. Inmates must pay $0.05 per minute for use of this system, and are permitted to print messages at a cost of $0.15 per page. (In many U.S. federal prisons inmates wages start at 12 cents per hour.) Sending a message to someone can cost up to $0.30. This service is also available in some state prisons such as Iowa.
Not all federal inmates have Corrlinks access, and inmates are may be barred from using the service if their particular crimes involved the use of a computer in any manner. The system does not allow inmates access to the Internet, and all incoming and outgoing messages are monitored. Emails are limited to 13,000 characters, and no attachments are allowed. (Attachments will be removed, which sometimes corrupts the rest of the message.) The content of the email may not "jeopardize the public or the safety, security, or orderly operation of the correctional facility".