Union spokesman Alain Duguay said there have been a dozen such incidents in northeastern Montreal since the beginning of the year.
One man usually distracts the carrier while the other approaches from behind and demands the keys, said Duguay, the local president of the Canadian Union of Postal Workers.
The assailants then use the keys to steal credit cards, passports and other important documents.
"With the keys, they can retrieve the mail that's either in the grey boxes or in a series of letter boxes," he said in an interview.
Some of the keys also provide access to apartment buildings.
Duguay said two of the attacks took place at community mail boxes, with the most recent incident occurring at the beginning of this week.
"What we're telling our members is to not resist, to hand over the keys and call the police as quickly as possible," he added.
"We're asking our members not to take any chances with their safety."
The union official added that one female carrier was targeted twice but that nobody has yet been injured.
Duguay said similar incidents occurred four years ago in the same sector and involved "organized" street gangs.
During those earlier attacks, a male postal worker was injured.
Montreal police spokesman Raphael Bergeron said an investigation is underway but that it's too early to link the incidents to street gangs.
Duguay is worried the arrival of community mail boxes across the country will make it a lot easier to rob the mail.
He said there have been more than 4,800 thefts involving community mail boxes in B.C. in recent years, mainly in the Vancouver area.
Canada Post is adamant about moving ahead with plans to gradually reduce home mail delivery despite court challenges and calls for a moratorium by some mayors.
Jacques Cote, who's responsible for the file, told The Canadian Press this week the Crown corporation has no choice but to go that route because of a continuing drop in letter volume.
He also said Canada Post is ready to defend its position all the way to the Supreme Court of Canada.
"We think we're on solid footing and that our position is legal," Cote added.