I presume there was a certain amount of mail that was local within the current region or within what would have been the sort region for Dryden, whereas the rest of the mail still would have been sent off and sorted in Thunder Bay for international and national markets.
Evidence of meeting #33 for Government Operations and Estimates in the 42nd Parliament, 1st session. (The original version is on Parliament’s site, as are the minutes.) The winning word was mail.