try restarting from the help menu > troubleshooting mode and reset toolbars when it is offered.

Thanks for your prompt reply, Matt. Tried that, didn't work. Now, after re-customising my toolbar I've managed to lose 'Mark' as wall. I suspect that my problem was caused by dragging an icon from the icon box to the toolbar, overshooting and dropping the icon on the line above. Has anyone else managed to mess it up like that? David Walker

If you had dropped it onto the 'Menu Bar' in error then it would display on the 'Menu Bar' - assuming you have the 'Menu Bar' enabled.

The 'reset toolbars ' would clear everything because it has been reset back to default. So, you would lose anything eg: 'Mark' from the Mail Toolbar. But after resetting you should be able to use 'Customise' again to put items back onto the Mail Toolbar.