Once the Plesk panel gets updated to 11.5.x , the horde gets an upgrade as well : to horde 5.
On horde 5, there might be a weird issue in which mails would be sent out fine, but any attachments would not be. You will not see any errors or warnings, however, the attachments will not be received at the other end.
The issue was , the temporary directory of horde had the wrong ownership of apache:apache. Change this to horde_sysuser:horde_sysgroup and things will be fine.
First check the location of temporary horde directory :
# grep -i tmpdir /usr/share/psa-horde/config/conf.php
Most of the cases, it should be /tmp ( the default value )
# ls -la /tmp/ | grep .horde
Find the ownership of .horde and check if its apache:apache
Change it to :
# chown -R horde_sysuser:horde_sysgroup /tmp/.horde/
This should solve the problem with attachments !