From the USA.gov Blog:
The U.S. Postal Service has established a unique Post Office Box to which the public may send condolences to those affected by the December 14, 2012 tragedy in Newtown, CT.
Those who wish to send expressions of comfort should address them to:
Messages of Condolence for Newtown
PO Box 3700
Newtown CT 06470
“We understand that there is an outpouring of support for everyone in the Newtown area, and we hope to make it easier for those who wish to send encouragement and messages of compassion to those affected,” said Kimberly J. Peters, Connecticut Valley District Manager for the U.S. Postal Service. She added, “Through letters and cards, families will be able to hold that friendship in their hands and to read those kind words at a time and place that supports their pace of healing.”