It's my default email manager. There isn't a single thing I can't do with it, whether it's the default version, the improved Betterbird version, or extensions like those on the Firefox browser.

For example, message filters – not yet available on Android because their takeover of K9 Mail is too "recent" – but super useful for sorting emails into the right folder.

With Filtquila, Regex lets me boost that, and Betterbird also lets me remove emails containing any elements (attachments, subject, etc.) from the search, which I also do in classic Thunderbird.