All encryption and decryption is done on-the-fly and completely transparent to you. You can also upload your friend’s public-key to the HushMail server using HushTools, and likewise, your friend can download your public key so that both of you can send secure emails back and forth.
Another cool feature that Hushmail offers is the ability to send encrypted emails to someone without them having to use any encryption software. If you both have a shared-secret (password), you can send someone an encrypted email that they can decrypt later using the shared-secret. [via Bauer-Power]