[Paul K]

You drag the dotted line that will be visible when you select "lock the toolbars" to unlock it (there will be a tick against the option if the toolbar is locked), you don't drag the icon itself, just drag the dotted vertical line across to the left so that there is enough room on the toolbar for the icon to show. How many icons have you got showing by the way? If there are too many the read mail one may not have enough space.