Current state:

Not that the important/unread filters do nothing right now, so I'll probably just hide the whole sidebar for the time being.

a bit simple.

Sidebar would be usefull also with the last folders where you move mails (office users use folder structure)

I'm a big fan when search show you some usefull defaults when you start search.

eg. you click search and get not only the search field, you get recent 5 search strings, recent 5 conversations, recent 5 users you wrote an mail, ...

the idea is to have something "artificial intelligence" (usefull defaults) when you click search and when you type manuel intelligence start.

The simpler the better is the mantra, right? ;-)

But yes, hopefully we can improve it over time to make it more useful.

So your suggestions would be:

• A list of most recently-moved to folders as filter
• Recent "items" (search strings, conversations, people you communicated with) on the initial screen below search, with the list of recent items potentially adapting to user behaviour.

I'm not sure how well those ideas would really work, but it could be interesting to work them out in a bit more detail, as well as collect some other alternatives.
I'd like to avoid cluttering the UI with too many things that nobody ends up using, but if we can find a couple of high value targets I'm not against implementing them.

In the long run I could see a certain amount of customization to be supported, so a user can e.g. add/remove "most recent X" widgets, but that's not something we can afford right now.

First simple than sidebar with filter to folders if someone don't use folder, no problem, than the sidebar is nearly empty. 3rd step can be some intelligence.