If you like to retrieve the Panther look of the toolbar instead of the crappy Tiger look it's very easy without the use of any external programs.
Select Mail.app in your applications folder and Ctrl-click to open a drop down menu. Select 'Show Package Contents'
Open Contents -> Resources
Find all .toolbar files (There are four of them) and open them.
Change all ' segmented = 1; ' to segmented = 0; '
This will alter the toolbar to use the old Panther icons . You the have add the the buttons and create the layout you want with ' Customize Toolbar ' menu option.