Version: (using KDE KDE 3.5.7) Create folder A, rename it (properties), enter dot as its name ("."), ok --> "unnamed". KMail should not accept this rename if this name is forbidden, user can search long for the name she/he gave.
See also bug 45773.
See also bug 148792.
This bug is still reproducible on trunk r948809.
I fixed yesterday in 4.4.x