If you can switch to IMAP instead of POP3, there's a line in this file:
/etc/e-smith/templates/home/httpd/html/horde/imp/config/prefs.php/135sent_mail_folder
That says:
// For Exchange, uncomment the entry below.
You would be able to create a custom version of the file specified and figure out how to specify 'Sent Items' instead of "sent-mail" for your sent folder.
You would probably also want to fix the folder name for "Trash" / "Deleted Items" in (a custom version of) this file:
/etc/e-smith/templates/home/httpd/html/horde/imp/config/prefs.php/165trash_folder
It might also be possible to do what you want by symlinking "Sent Items" to "sent-mail" for each user...
Finally, I'm pretty sure each user can pick different folders for 'Sent' and 'Trash' manually in Horde.
If you're stuck with POP3, you may be able to create a rule in Outlook (since the 'Sent Items' folder is local on each outlook user's workstation) that scans the 'sent-mail' folder on the server and downloads a copy of each message (although, to do this you'd have to setup an IMAP connection to the mailserver anyway).
Or (I used to do this for Blackberry users), you may be able to configure Horde to BCC a copy of all outbound email to the user, and configure a rule in outlook that identifies these BCC'd emails and moves them to 'Sent Items'. This has the added advantage that everything 'Sent' from Horde shows up as 'unread' in the Outlook Sent Items folder (in case users need a hint about when they wrote a given message).