Mail - IMAP - Archive?

Hey;

So I'm confused. Our small business all switched to the iPhone when the 3G came out in Canada. We were previously all running on POP mail. When I transitioned everyone over, I moved us to IMAP. Well the inevitable has happened, our server is now full.

So... Since we need to keep all of our emails, I set a rule "if message received is older than 30 days move to folder (Archived-Received)".

Well now OS X is just sitting there downloading 1300 messages. Then when it finishes I click my inbox, and my messages flash from 45 messages to 1345 ... and then back to 45. My archived folder is empty... and then mail starts to download the 1300 messages again?

Little Help?