On 02/07/2005, at 5:04 AM, Severin Crisp wrote:

Where does mail keep received attachments?

In 10.3.9,

attachments aren't separated from the message, unless you do it manually from within Mail.
The pathway to the inbox is:
~/Library/Mail/POP«account name»/INBOX.mbox, if you want to inspect file size to confirm this.
Attachments are therefore deleted when you delete the message.

One space-saving app in OS X - at least ;-)