Using filters and rules and downloading the msg, and drive space

I've tried this both on Outlook '03 on WinXP and Kmail on Fedora Core 3, and the same thing is happening, so I take it it's platform indipendant behavior.

What I'm asking is, what's actually happening with the message when the following happens?

I set up the client (Outlook-Kmail) for IMAP. It opens the root mail folder, well and good. When I set up a filter to move a message to another folder, or when I manually move a message to another folder, the message goes in, then is either marked as deleted or actually deleted (Outlook and Kmail respectively) in the original folder.

So, is the message at that point being downloaded OFF the server?

If I then check the account with a POP3 client, it downloads all the messages...except the one I filtered-moved. Which leads me to be certain the filter-move downloads it to the PC.

If this is the case, is there some way to be able to use folders and filtering with IMAP without having to download the msg, which in my opinion defeats the purpose of using IMAP in the 1st place over POP3.

Also, one of the reasons I want to use IMAP is because the client PC's here are reaching multi-gig e-mail sizes. (They have to use Outlook, so we're talking PST files...which become more unstable the larger they get.) buttuming the above is-becomes moot, and we save all the e-mail on the server....I just realized that now means the users' mail spools will become huge. Is that a bad thing? WIll that be a problem to eventually have 4,6,10 GB mail spools? (It's a RedHat AS2.1 server.)

Thanks for any buttistance! Liam