How do I de-encrypt an email?

I received an email from someone and it’s obviously encrypted somehow. I think I know the key to de-encrypt it so that’s not my issue. I just don’t know how to de-encrypt. What program do I use? I have no idea what program they used.

And no, I can’t contact them and ask them what program they used. I just can’t. To make a long story short…well, no.