By default, Horde will strip the attachments from your sent messages in your sent folder. The attachments will still be sent it just won't save a copy. This is an attempt to save space and prevent sent mail from taken up too much space on your hosting account.
To enable Horde to save a copy of the attachments, click on the Settings icon (cog icon) > Preferences > Mail. Next, click ‘Sent Mail’.
Click on the drop down menu under 'Save attachments in the sent-mail message?' and choose 'Save Attachments'. This will keep a copy of the attachments going forward.
If you have multiple email accounts, the same change will need to be made for each account.
If you require further information/assistance on this, please Open a Ticket and a member of the team will be happy to assist further.