I can't comment on oakmail, but I can tell you how i'm doing it for my project (decentralized mailer/messenger, look for my post on this page). But basically yo

I can't comment on oakmail, but I can tell you how i'm doing it for my project (decentralized mailer/messenger, look for my post on this page). But basically you perform a lookup (X address = Y privatekey). The problem is when your users don't want their pubkeys to be public. In a nutshell, I create a new keypair using the recipients address as the seed for the keypair. Addresses in my system are similar to bitcoin addresses (I also use elliptical curve encryption) so the addresses have embedded hashes and checksum of the public key. This means that pubkeys are encrypted and only people with your address can retrieve your pubkey.