Members of the Hazardous Area Response Team were tasked to the Royal Mail depot in Mallusk on Wednesday after white powder was discovered inside a package.
The Hazardous Area Response Team (H.A.R.T.) operate as part of the Northern Ireland Ambulance Service.
H.A.R.T. members are specially trained to provide medical care to patients in hot-zones of hazardous environments.
A Royal Mail spokesperson said safety procedures were followed and that the incident did not had an impact on the processing of mail.
"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 Northern Ireland Ambulance Service received a call at 10:22am following reports of an incident at Enterprise Way, Newtownabbey.
Two members of the Hazardous Area Response Team and one Rapid Response Paramedic were despatched to the incident.
A spokesperson for the PSNI said: "Police are currently attending a report of the discovery of a suspicious package at the postal sorting office at Enterprise Way, Newtownabbey.
"Northern Ireland Fire & Rescue Service are in attendance.
"There are no further details at this stage."