Export email from Horde.
1. Login to Horde Mail. Click "Folders" icon at the top.
2. Select the folder you wish to export.
3. From the drop down menu select "Download" or "Download [.zip format].
4. Click OK to download email to local PC.
Install the tool, it is easy to convert .mbox to .pst.
After covert the file, access to below link to understand how to import pst file to Outlook.