Cover the spilled contents immediately with anything (e.g., clothing, paper, trashcan, etc.), and do not remove the cover.
It is very important that people do not pass around the envelope, show it to others in the room, carry it to a different area, etc. Better to err on the side of caution rather than risk exposing more individuals to the suspicious substance.
Let law enforcement personnel evaluate the situation. Do not try to do it yourself or with co-workers.
Then, leave the room and close the door, or section off the area to prevent others from entering (i.e., keep others away).
Turn off the HVAC (air conditioning) system.
Wash your hands with soap and water to prevent spreading any powder to your face.
Remove heavily contaminated clothing as soon as possible and place in a plastic bag, or other container that can be sealed. This clothing bag should be given to the emergency responders for proper handling.
Shower with soap and water as soon as possible. Do not use bleach or other disinfectants on your skin.
If possible, list all people who were in the room or area, especially those who had actual contact with the powder. Give this list to the local public health authorities so that proper instructions can be given for medical follow-up, and to law enforcement officials for further investigation.