Server Admin > Internet > Mail > Configure Mail Service > Protocols > IMAP Options…
Check "Allow IMAP Administrator Access." Choose a port (default is 626).
Click "Save." You can quit Server Admin now.
Mail > Preferences > Accounts > Add Account
Fill out all the information for an admin user with IMAP email access enabled. In Mail 1.2, you want to go to the "Advanced" tab and change the port number to the one selected above (default is 626) and make sure authentication is set to "Password." Look for similar options in other email clients. The important items are to have your IMAP connection take place on the port specified in Server Admin, and that the user be an admin user with email access enabled on the server.
Now, if memory serves me correct, you will need to transverse the account in the Accounts Drawer in Mail.
(Account Name) > Hosts > (name of server)
Here, you should find all the emails that have yet to be delivered. Start deleting.
Of course, as soon as I finished typing all that, I found the relevant
Knowledge Base Article. Have fun.