To set up folders for your IMAP account:
1. Log in to your webmail account via Horde (http://webmail.mydomain.com - replace mydomain.com with your actual domain name).
2. Click on Options, then Mail.
3. For Sent mail, click on Personal Information, then click the check box for "Save Sent mail", and click "Save Options".
4. For Drafts, Trash, and Spam folders, click "Server and Folder Information", and click the check box for "Use IMAP folder subscriptions", then click "Save Options".
5. Now that the folders have been created on the server, you can set up your mail program (Outlook, Apple Mail, etc) to use them for your Drafts, Sent, Spam and Trash. (For Apple Mail, first go to Mailbox > Synchronize All Accounts. Then highlight the new Sent folder you just created, and go to Mailbox > Use This Mailbox For > Sent. Repeat this for the Drafts, Spam, and Trash folders).