Email using different versions of outlook and on Smartphone

A few of my employees have desktops and laptops and, unfortunately the desktop is using Outlook 2007 where their laptop is 2010 or even 2013. The question though is why, when they delete an email from their laptop is it still on the desktop? And vice versa. The problem also goes with both iPhone and Android smartphones as well, when you delete the email from your phone, it still stays as new on laptop and desktop? Could use some help here. Thanks all!