Canadian Union of Postal Workers is filing a constitutional challenge on the government's back-to-work legislation.
Meanwhile, a big name is being brought in to try to resolve the contract dispute.
Elizabeth MacPherson, a former chair of the Canada Industrial Relations Board, will have up to 14 days to try and mediate the contract impasse between Canada Post and the postal workers' union.
Canada Post says it will "fully participate" in the mediation process.
The union has also agreed, reluctantly, in a bid to avoid binding arbitration.
