LockedEnvelope makes clear that the service they provide is on your shoulders if you forget your key/answer:
"We hash your answer in our database, which means that we can't recover it if you lose it. It also means that if someone broke into our server and looked at our database, they can't recover the answer. The secret message is encrypted using your answer, which, again, we don't have."
For now, according to a company spokesperson, the service will be free, with the possibility that mail with attached files or other bandwidth-intensive materials may carry a per-piece charge.
A free demo -- "Make Your Own Envelope" -- is available here.
Early days yet to say how effective this will be, but at the moment it's free and might well be an effective solution for securing mail between you and frequent recipients such as your remote workers.