Some Christmas deliveries in the north west may not arrive until January because of the bad weather, according to the local postal workers union.
Charlie Kelly from the Communication Workers Union said it could be next month before the backlog is cleared.
He said the bad weather made it well nigh impossible for postmen to get into many areas.
"We're out in conditions polar bears wouldn't go out in."
"It wouldn't matter how many staff there are, it's about the conditions.
"Royal Mail cannot afford to send them into places where all you hear all day is accident here, accident there.
"We have to walk in it, and that's rural areas, towns, and up steep hills.
"Sometimes it's just impossible.
"We will have the sorting office in Derry cleared by Friday, but people need to be aware that there are Christmas cards and parcels sitting in other offices which just aren't getting to us."
On Monday, Royal Mail said it would add extra evening deliveries in Northern Ireland in the run-up to Christmas.
A spokesperson from the company apologised to customers in the north west who had experienced delays to their deliveries.
"Our postmen and women are pulling out all the stops to get the mail delivered in spite of very challenging circumstances.
"We will continue to do whatever is possible to keep the mail moving but in some cases it is not possible to do so because of the weather.
"A range of measures have been put in place to ensure deliveries reach people as quickly as possible."