(Continuing from my last post.) My second project is still in progress. During a discussion of signing and encryption on the mutt mailing list, Florian Lohoff mentioned an idea that I found interesting. (He had actually mentioned it in the past too.)
Currently, mutt has no way to automatically turn on encryption if you have a key for all message recipients. You can set up hooks that turn it on for a single email address, but when you email multiple people it doesn't work properly. If one person has a hook set up, it will turn encryption on despite the fact that some recipients don't support it. This seems like a useful feature that could lead to more wide-spread adoption of encryption (for mutt users).
At first I thought about an editor plugin that would add/remove a "Pgp: E" flag based on recipients. Another idea would be a wrapper script similar to the one mentioned in Aaron Toponce's blog post. The problem is the Pgp header can turn encryption on, but it can't turn it back off. So if you add people after first editing the message, you have to manually turn it off.
That led me to investigate putting it into mutt itself. Thus was born a six part patch set implementing "opportunistic encrypt". Hopefully the mutt developers will like the patch and be willing to consider committing it. Stay tuned!