For the second time in recent days the main sorting office at Mallusk, outside Belfast in Co Antrim, was evacuated after a telephone warning that a bomb had been placed in the building.
Some 200 staff were out of the premises for more than three hours before the alert was declared a hoax after a full search.
The disruption is believed to have been caused by dissident republicans who have caused chaos on Northern Ireland's railways with bomb and bomb hoaxes in the past week - as well as stopping the mail earlier in the week.
Earlier, a pipe bomb was defused early today at a house at Annalong, Co Down.
British Army bomb disposal experts were called in yesterday after the occupier of the house on the Glassdrumman Road discovered the device and raised the alarm.
The RUC was trying to discover a motive for the house being targeted.