Magnotta has pleaded not guilty to the murder and dismemberment of Jun Lin, a student at Concordia University.
Montreal police received a call about 5:20 a.m. ET Tuesday after the package was found at the St-Laurent facility.
Workers immediately called police and put the building on lockdown. Fifteen employees were quarantined for a couple of hours and one person was treated for shock at the hospital.
Initial reports described the contents as a powder, but later said only that it was a "harmless substance."
The perimeter around the building was lifted just before 8 a.m. and employees were allowed to return to work.
The facility on McArthur Street is the largest postal sorting station in the province.
A pale-coloured powder was also found in the bottom of a container at a sorting station in Sainte-Julie, a southern borough of Montreal.
This situation was also deemed harmless by officials.
This is not the first time Canada Post workers in Montreal have dealt with suspicious substances sent by mail.
A month ago, nearly two dozen government officials and media organizations received packages containing a strange but harmless powder.
Canada Post officials say workers have to be aware of the risks and should not take chances when finding suspicious substances in the packages they handle.