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.
A Royal Mail spokesperson said safety procedures were followed and that the incident did not have 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.”