Comment 7 for bug 331478

Sebastien, you are right, but I haven't found any way to reproduce the issue (until now it's random). However some days ago I synced all my emails with the workaround I spoke in my previous comment and, at every new email, I'm trying to check if it's downloaded or not.
When I have some time I will try this issue in newer version, as asked by Pedro and by you.