As it stands now, large folders are an issue both for filtering and the file size limit.  I'm sure v5 will address the size issue but selective folder filtering on the read status would be a nice addition provided checking the status results in significantly improved efficiency.

Would it be possible to allow general filtering sets that can be attached to folders to have three options:

Apply filtering to read messages
Apply filtering to unread messages
Apply filtering to all messages

I have an account set up that receives notification messages from our security software and OS processes. I have new mail filtering rules set up that generally sort the messages into dedicated folders when the account is opened, and have general filtering rules attached to those folders that allow fine grained filtering.

Some of these folders have thousands of read messages and when I open that folder, the filtering process can take 30secs or longer. I assume this is because the filtering rules are being applied to every single message. From my point of view it would be useful to be able to configure an option to apply the rule to unread messages only. If I need to change the rule it may need to apply to all messages.

Would anyone else find this useful? The amount of time saved in the long run would soon add up.