If you read the Internet Diet article, you probably remember that sending or forwarding chain letters via email is not an authorized activity. On the surface, it may appear that there is very little, if any, cost associated with sending a chain letter to several people using your office email system, especially if all the recipients are NIH employees. The real issue is that NIH email and Internet services are for government use in support of the NIH mission. Use of NIH network systems to send and forward chain letters and other inappropriate messages is prohibited. They are also illegal if they ask for money or anything else of value.
Most email systems store the incoming and outgoing messages on a server hard drive.
Storing a thousand messages takes up a lot of hard drive space. Transmission of the messages reduces the availability of the system for other messages that are official business, and could require additional capacity to provide adequate facilities to handle all of the traffic.