Maybe this is old news, but we've been getting a a lot of mail like bills and such, with a bunch of them supplying those free reply envelopes with postage paid within Canada. So for kicks I took a few and taped over the address and sent them to some other relatives in Calgary and they received them!! I've still been skeptical with using them for any important mail, but the wife thinks it was awesome and used them for some mail in rebates that I thought we would never see again and Bingo, just the other day recieved our $30 rebate cheque.
So keep those free postage paid envelopes and tape over or mask or whatever over the original address and they are good to go within Canada. Not sure if any laws are getting broken here