No return address, or one unknown to the recipient. Minimize handling, and gently place the item on a stable surface. Isolate the package. Evacuate the immediate area. Do not put the package in water.
Water can cause an explosion.
Do not put the item in a confined area, such as a filing cabinet. The explosive may be powerful enough to blast the container apart and produce dangerous shrapnel. If possible, open windows to vent explosive gasses and reduce glass shards in case of detonation. If you are truly suspicious of any package, do not be afraid of embarrassment that you may be wrong. Call the authorities, especially your local Postal Inspector.