One spanner in the works here is that not all applications with mail preferences are on Modular Transactions yet. I think these are missing:

• Slowvote
• Ponder
• Phriction
• Pholio
• Fund
• Conpherence, technically, but it's not on the same system anyway.

At some point, we probably need to copy users' settings to the new system -- at least, mostly -- since I think we'll get a lot of pushback if we make everyone go set everything up again from scratch.

For now, I'm going to do this:

• Put the new system in place, but as a prototype, and not link it in the UI yet. The implication is: if you use it, your settings may be changed in the future by the eventual cutover/migration. (But: I think we can just add rules, not overwrite anything, so cleanup won't be too bad.)
• Once it works we can either convert the missing applications, do a partial cutover, or do a full cutover and just accept that settings for these applications won't be customizable until they're updated. Slowvote and Fund are probably easy to update but the others might be more involved.

Just to clarify, it is good to be moving all the apps to modular transactions? This task is my primary motivator to cleaning that up.

Yeah, that directly helps with this. I think I have a pretty good plan for the actual mail part that isn't too crazy complicated, but the cutover/migration bit will be much simpler/smoother if we get things swapped over -- at least, most of the way -- ahead of this.