Deleting attachments

How do I stop Mail from retaining attachments of deleted email messages?

Today I found 50 mb of attachments left over from deleted messages in several Library Sent Mail folders like this:

~/Library/Mail/Mailboxes/Sent Messages (myAddress1).mbox/Attachments/

Each folder contains nested numbered folders holding attachment files from deleted messages.

It was my understanding that when a message is deleted, any attachments with it are also deleted.

I keep only 5-10 messages in my Sent mainboxes, and I do Erase Deleted Items and Rebuild (mailbox) daily.

These are all IMAPs, and in Preferences/Accounts/Advanced, I have "Keep copies of messages for offline viewing" set to "All messages, but omit attachments". Is that the problem?

Thank you.

MBA 11-inch 3,1 – OS X 10.6.8 – Mail 4.5