It's not really possible to send encrypted messages to people who aren't already using OpenPGP, since you need to get their public key, before you can encrypt the message. You could presumably encrypt it with some symmetric cipher ahead of time, then send them the encrypted junk and say, "Send me a PGP key and I'll send you the passphrase for the message!", but I dunno that anyone's going to do that.

That said, pretty much every message I send to someone who doesn't have a key on the keyservers includes, "Hey, send me your PGP key, I don't do plaintext."