Some emails create an enormous tooltip window which crashes compiz (compiz (core) - Warn: Exceeded max texture size) when clicking on the title to expand the contents. (see here for example https://bugs.launchpad.net/ubuntu/+source/compiz/+bug/599723 ). Admittedly this is probably a compiz bug but there is no progress on the bug - is it possible to disallow checkgmail from producing the large tooltips?
It can be very annoying if you have 100 windows open over virtual desktops since running compiz --replace returns the windows to the 1st virtual desktop and messes up the workspace. Also, there is no way of telling which email will produce this behaviour until you click it