Hello,

All the files under RoboMail can be deleted.

However, if you're still running the software, the program will be locked to avoid any abnormal situation. You may check that the RoboMail icon is in the System Tray if it is running.
You should quit the RoboMail before uninstall the software.

If you uninstall the software while it is running, some files may be locked and cannot be deleted. In your case, you can reboot your machine and the files lock should be released.

FYI, the files of RoboMail are located into two place in your machine.
The core program are stored in C:\Program Files\RoboMail
The data and other files are stored under #YOUR_USER_NAME#\My Documents\RoboMail