You can email a prisoner by logging onto www.emailaprisoner.com. You will find all the information on this website. Once you have registered you will also be able to gain access to the text messgae service. He can't send one back but it is an easy way to keep in contact.
You can both keep in contact by writing to each other. You can send him stamped addressed envelopes or he can buy them from the canteen.
HMP Chelmsford 200 Springfield Road Chelmsford CM2 6LQ
If he has credit on his pin phone then he can call you. All numbers that he wants to call has to be approved by security first.