If we right-click on an IMAP folder and go to 'folder properties > retrieval', one can choose to either retrieve full messages, or retrieve on demand. It would be useful to be able to retrieve full messages for recent mails, and retrieve on demand for older ones. For people with large mail inboxes, retrieving everything just to have access to perhaps the last 30 days of mail offline seems like a waste. Ideally, there would be a third option which would retrieve full messages only for mails in the last X days (where X could be user-configurable).

Apologies if this already exists as an option somewhere. I couldn't find it.