A letter bomb was found overnight at the Royal Mail sorting office in Mallusk.
Part of the depot had to be evacuated last night as Army bomb disposal officers dealt with the device contained in a suspicious package.
The security alert ended in the early hours.
The attack was described as “reckless” by Northern Ireland Secretary of State Theresa Villiers, who added: “Once again these reckless and cowardly individuals have demonstrated complete disregard for postal workers and local community.”
The object has been taken away for further examination.