It's a different country, different postal service.
that being said... I've had a few SASEs with Canadian postage travel back to me from the States. I sent to Rob Ducey and Brad Arnsberg c/o the Jays and expecting the mail to come back from Toronto I put Canadian postage on the SASE... but they came back during Spring Training post marked from Florida.
I think sometimes you get lucky that whoever is processing it just sees a stamp with a number on it that corresponds to "enough postage" and he/she doesn't realize it's another country's stamp (doesn't recognize the stamp but maybe figures it's new).