Its not used nearly as much as it used to. Right now its a holding spot for domain logos, as well as temporary holding spot for files being uploaded as attachments or file storage before they are finalized and moved to their final spots. So if App_Data is not working I'd probably expect uploading to file storage or attachments not to work.
I think we fixed this though in our previous build so it's interesting your still having issues with it and doing chmod like that should have eliminated any permission issues with that.
Edit: Remembering now I think part of the issue is that it was being put in /opt/smartermail/App_Data which was wrong.
It should now be inside /etc/smartermail/App_Data
Maybe you can check that folder and ensure its there with the correct perms?
Trying to keep /opt/ for the binary files, and /etc/ for the system config files and stuff like App_Data, and /var/lib (if you choose to use it) as the data folder.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com