The leaders of Montreal, Laval, Longueuil and surrounding suburbs want to obtain intervener status in a Federal Court lawsuit filed by the union representing Canada Post employees.
Canada Post's decision was made without consulting municipalities and will hurt large numbers of seniors and people with reduced mobility, the mayors told a news conference.
Montreal Mayor Denis Coderre said cities should be "full partners" with the Crown corporation on plans to modify home-delivery services. He went so far as to threaten to make mail delivery a federal election issue this fall.
"We are part of the deal," he said. "If you don't want to make this an election issue — (install a) moratorium."
Canada Post has said it is committed to moving ahead with its plans — originally announced in December 2013 — to gradually reduce home mail delivery and install community mailboxes despite court challenges and calls for a moratorium by Coderre and other mayors.
The corporation says it has no choice but to go that route because of a continuing drop in letter volume.
"The fact that digital substitution is decimating traditional mail is undeniable and Canada Post has an obligation to preserve the postal service for all Canadians without being a burden on taxpayers," Canada Post spokeswoman Anick Losier said in an email Thursday.
Losier added that Canada Post is "working with 90 municipalities across the country" to select the best places for the community mailboxes.
She said the corporation is also working with affected residents and "moving boxes to accommodate neighbourhood requests."
Marc Demers, mayor of Laval, which is just north of Montreal, dismissed Canada Post's claims it is listening to communities.
He said his council passed a resolution in October to collaborate with the Crown corporation to minimize impacts of the reduced service, but did not find a willing partner.
"The result is that Canada Post works alone," Demers told reporters Thursday. "The communication goes one way and they don't listen to our recommendations."
During 2014, Canada Post converted 100,000 addresses that had door-to-door delivery to community mailboxes. In 2015, it plans to convert about 900,000 addresses.