The federal Crown corporation says the loss comes in the midst of a "historic shift" to digital communications that has eroded mail volumes at an "accelerated pace."
As a result, it is in the process of consultations to decide how the company can change with the times, as fewer people continue to send mail and use post offices.
Canada Post said this "multi-pronged transformation" is part of an overall effort to "avoid becoming a financial drain on taxpayers."
The $104 million loss before taxes was heavier than a loss of $102 million in the same quarter a year earlier.
Transaction mail, which includes letters, bills, and statements and accounts for half of Canada Post's revenue, was down by 51 million pieces or 6.3 per cent in the quarter, compared with a year earlier.
However, Canada Post said its parcels delivery business grew due to the popularity of online shopping. In Canada, volumes in the second quarter were up by 5.1 per cent, compared with a year ago.
Canada Post said it continues to cut costs by making changes to collective agreements with its workers, who are members of the Canadian Union of Postal Workers. The changes include reduced wages for new hires, the elimination of banked sick days, and a one-year-wage wage freeze for new hires beginning 2015.
It is also shortening hours at its less-in-demand retail postal outlets, and consolidating mail processing.
The company said the second-quarter loss means it is on track to record a substantial loss in 2013.
Canada Post has been grappling with a persistent drop in many forms of mail as a result of newer technologies such as electronic mail and online banking.
Besides Canada Post, the group includes the Purolator courier service and other subsidiaries.