I have an idea which is not quite what you asked but you might find as an improvement. At the bottom of the 'Spaces Toolbar' in the far left corner of Status Bar is a left pointing arrow. If you click on that arrow it close the 'Spaces Toolbar' and puts an icon called 'Open Spaces Menu' on the far right of the 'Tabs'. Right Click on the 'Mail Toolbar' and select 'Customise' Now you can left click and hold down on the 'Open Spaces Menu' icon and drag it down onto the 'Mail Toolbar' - I put mine on the far left of 'Get Messages' button. Then I used the Customise' window to add a vertical line between 'Open Spaces Menu' icon and 'Get Messages'. You can then use the 'Open Spaces Menu' icon as a drop down menu. OR use the 'right pointing arrow' in far left corner of Status Bar to quickly enable the Spaces Toolbar again. That left or right pointing arrow is the quick display/hide icon.