Horde is sending attachments with size of 0 bytes
A problem related with the cpanel-horde user quota. The fix, edit the quota for the user cpanel-horde:
edquota cpanel-horde
Change the soft and hard value for anything above the actual quota.
Disk quotas for user cpanel-horde (uid 32008):
Filesystem blocks soft hard inodes soft hard
/dev/hda5 154116 1536000 1536000 432 0 0