Northern Ireland Fire & Rescue Service are in attendance.
A spokeswoman for the Fire Service said: “We are currently assisting Police with an incident at the postal sorting office at Enterprise Way, Newtownabbey. The incident is ongoing.”
A Royal Mail spokesperson said: “Royal Mail can confirm that white powder was found in an item at the National Returns Centre (NRC) at Northern Ireland Mail Centre in Mallusk this morning.
“The safety of our people is of the utmost priority and in line with our policy and procedures, the emergency services attended and the area where the powder was found was cordoned off. We are assisting the emergency services with their investigation.
“There is no impact on the operation of the mail centre and the processing of mail.”
The NRC is where staff have authority to open mail. This enables them to try and return all unclaimed items to their sender.
They also attempt to reunite undeliverable items with the intended recipient, or return them to the sender.
There are no further details at this stage.