During the first two weeks of recruit training, recruits will send home a letter stating they've made it to MCRD San Diego and will provide an address for which to receive mail.
Here's is an example for recruit mail:
Recruit John E. Doe
1st BN, Alpha Co. PLT _ _ _ _
36001 Midway Ave
San Diego, CA 92140-(plus platoon #)
If a message is required to notify a recruit of an emergency situation at any time during recruit training, please contact your local America Red Cross or please visit the American Red Cross Emergency Communication Services web page, to have an official message sent to MCRD, San Diego. In order to process the message quickly, please provide the recruit's name, social security number, company and platoon.