Got it to work: mail form support, after upgrade isymphony is definitely more stable, still a bit laggy sometimes, but better.
The app stores it’s cache in these folders:
Windows:
C:\Users{user}\AppData\Roaming\iSymphony Desktop\Cache
Linux:
/home/{user}/.config/iSymphony Desktop/Cache
OS X:
/Users/{user}/Library/Application Support/iSymphony Desktop/Cache
By clearing the contents of these folders you will clear the cache.