I’m interested in seeing secure email mode implemented.
A single site wide admin setting to enable/disable secure email mode is a good starting point but perhaps a bit draconian for most sites including ours. We have many users accustomed to using email to participate in discussions and we also have many topics that are public or, if private, not worthy of this level of security. It would be a shame to lose all that - though on the other hand it also might also lead to more engagement since people would be forced to start logging in.
Here are some other possible ways to bite this cherry 
for your consideration:
- user preference - allow users to opt for secure email mode themselves, separately for PMs and posts. This is handy for users who might be feeling paranoid about the security of their own email mailbox and want to control what lands there.
- category security settings - allow admins to set up secure email mode independently for each category. Then we educate our users on where to put secure correspondence.
- break out site setting to enable/disable secure email separately for public categories, private categories, and private messages
- composer - add secure email option directly to the composer, so user can decide to keep a specific message or post secure. (probably overkill)