Wrong address on the envelope, what am I to do?

If I were receiving your letter, the envelope would soon be in the trash and I would reply to the address given in the letter. But if it's worrying you, resend the letter today with a note about the correct address. Also, it's not uncommon to send the same letter by both First Class and by Registered Mail. I don't know exactly why people (law firms in particular seem to love to do it) do this — maybe it's just a belt-and-suspenders thing, or maybe they're covering in case the Registered one takes longer — but I've had it happen to me several times. Exact same letter, two copies, one sent certified or Registered and the other just dropped in regular mail. Where I work (not a law firm), it's to prevent the recipient from claiming that they didn't get notification of [x] action. If we just sent it certified, the recipient could refuse to sign for it, or fail to pick it up from the post office, and have a reasonable claim that they were not notified. So some things also go by certified and regular mail, to cover our asses.

If you're that worried, resend the same letter with the corrected address.

Stop worrying: Nobody looks at or uses the return address on an envelope unless there's nothing else to go by. They'll use the one on the letter. If Canada Post is like the US Post Office, you can't re-direct from an arbitrary or false address to a real one: you need to prove you're a valid recipient at the old address. If you are tremendously concerned, though, what you can do is send a second registered letter tomorrow (two days after the first) that reads simply: "I included the wrong return address with my previous letter. Please use the address below to reach me: [ADDRESS]." Unless they have already responded (within two days?), that will work. And if they did respond already, at least they'll know they just sent something to the wrong place and they can deal with it.

Seconding meadowlark lime, plus if you have the correct return postal code on it anything sent to you in return should get back to you. But I'd send another ASAP just to be sure. Even if its just to notify the recipient of the correction.

Unless there is something very peculiar about Canada Post's Registered Mail service that I am unaware of, I don't think this will matter. The only situation where the return address on a mailpiece generally matters is if the piece needs to be returned due to being undeliverable or refused. As long as one of those two situations don't occur, you can pretty much put "Sandy Claws, North Pole City" as the return address and (although this is not recommended practice and may make your recipients wonder whether you've cracked) the mailpieces will probably get through fine. It would be extremely strange for a person to look at the return address on the outside of a package or envelope and use it for return correspondence, in preference to the one actually on the letter. (By "extremely strange" I mean that it would be wrong, as far as I'm concerned.) The only time I'd use an envelope address for return correspondence would be if there's no inside or letterhead address (this is sometimes the case with cards or personal notes). Of course people do dumb things all the time and there's no guarantee that somebody might not use that address, but I wouldn't worry about it.

Also, it's not uncommon to send the same letter by both First Class and by Registered Mail. I don't know exactly why people (law firms in particular seem to love to do it) do this — maybe it's just a belt-and-suspenders thing, or maybe they're covering in case the Registered one takes longer — but I've had it happen to me several times. Exact same letter, two copies, one sent certified or Registered and the other just dropped in regular mail. So you could do that if you want to make sure, and at worst it would make you look overly thorough.

I wouldn't worry about it, but then I don't know how important this communication is to you. Chances are very good that your correspondent is going to use the address on the letter, not the one on the envelope. Does your correspondent have some way to contact you if their letter is returned as undeliverable? What is the worst case scenario?

As long as the postal code is on there, they'll know what you meant.

If you kept a copy of the tracking number you can check up on it's status https://www.canadapost.ca/cpotools/apps/track/personal/findByTrackNumber?execution=e1s1.