Right-click anywhere in the toolbar and select ‘Customize’. In the window that appears, remove and add items or change their order by using the ‘Add’, ‘Remove’ and Arrow buttons.

In versions newer than eM Client 7.1, you can also customize the Mail toolbars centrally in Menu > Settings > Appearance > Toolbars.

Read more in our blog: Customizing user interface in eM Client and in the Documentation: Toolbars