there's a difference between using mail with 0-400 msgs per day and using mail with >400 msgs per day.
the former is the category where the mainstream emailagents are ok. but for larger number of mails there are are performance problems and mail-handling-inefficencies that criple one's ability to handle mail fast. (these inefficiencies are mostly by design and not bugs)
i for one use claws-mail, but am in the <400msgs per day category.
p.s.: and plz! don't mention outlook, because most people here don't even know what it looks like.