In Thunderbird you can do a complex search with
strg+shift+f, but just for each account. Besides, there is a global search field in the top bar where you can search for keywords in all accounts.
But I would like to use the complex search of
strg+shift+f over all accounts. I would also like to create a saved search for that search.
The background is, that I usually tag all my emails and feeds that I need to work on with 1 (important). And I don't want to check each account for unsolved messages.