I've just completed testing the next generation of my kuvert tool: Version 2.0.0 is out here and has just been uploaded to debian Sid. It's full of Nice New Things that make kuvert more useful, the most notable ones being:

• inbound SMTP support
  You can tell kuvert to listen on localhost on a port of your choice for inbound messages. (This absolutely requires ESMTP authentication as pointed out in the manpage.) Benefit: any garden-variety mail user agent can send via SMTP, which means it can interoperate with kuvert. You don't have to bother with the submission wrapper anymore (but it is still available of course).
• outbound SMTP support
  Kuvert now can speak SMTP to any server of your choice. No more need for a local MTA installation (unless you prefer one, in which case kuvert will work like before).
• support for gpg-agent

There are also quite a few other goodies, but I haven't cooked up a good changes document yet; You'll have to read the manpage.

Kuvert version 2.0.4 has been released. New feature: kuvert now supports SMTP Authentiction for submitting your outbound emails to an MTA (No TLS/SSL yet).

Sources here, binaries at the Debian mirror of your choice.