Ask HN: What are you missing in existing email clients?

I'm wondering both about features and quality. Personally, I dislike most of the desktop clients not for a lack of features, but for a lack of quality. Too often, UIs are clunky, or IMAP syncing doesn't work well, or search is dissatisfactory.

I should not be priming you more, but just ask: What features and qualities are you missing? What would make you try another client? What are absolute must-haves in terms of features?