Comment 2 for bug 1127444

I suspect this issue to be related to Unity3D.
If I reboot and choose another window manager (Unity2D or Gnome classic), the problem does not occur : alt gr does not select an address from the suggestions, and you can safely type the @ symbol.

Another strange behavior is that the problem disappears if I close the session, switch to another window manager, close the session again, and switch back to Unity3D.
It looks like the bug occurs only on Unity3D, and only if it's the first window manager after boot.