Rob Griffiths' Macworld OS X Hint Trim Mail attachments this week was about trimming attachments in Apple Mail. You can also trim attachments in Entourage using a script.
There are two scripts available to remove attachments from incoming mail.
Friendlier Claris It! By: Rob Buckley
Save Attachments 2004 By: Paul Berkowitz
Both scripts will save, remove and link the saved files to the message, so you can open the file directly from the message.
You can also remove your outgoing attachments with am Outgoing rule. This is assuming your outgoing attachments are stored elsewhere on your computer. No need to have them duplicated and filling up your database.

Comments (2)
This article is specific to Apple Mail app.. I know there are a few scripts out there for doing something similar in Entourage, but I would like to be able to schedule an Applescript to run a rule I've already setup that will look at my sent items fold (and/or my inbox) on an Exchange server, identify only those messages with attachments, make a copy of those messages to my local sent items (or inbox) folder, then remove the attachments from the server to save on server space. That way, I have a copy of the original message with the attachment for client tracking purposes, but am not contributing to server issues by leaving LOTS of large, graphic file attachments on the server. But I've yet to find an Applescript (or at least a template) that will let me simply execute an existing rule. Anyone?
There is no one rule to do what you want AFAIK. You might want to try the script AutoArchive Exchange Account to "On My Computer"
You should then be able to run a rule to remove attachments from your exchange messages once they are archived to your computer. You could do this manually say twice a month. Set a Task to remind you.
I don’t have the skills to be specific, but a schedule that runs an AppleScript could run the rule, or an Automator action might be able to do this automatically.
Rule to remove attachements:
Criterion:
All Messages
Attachment: Exists
Action:
Remove Attachments