Mail groups allow you to add multiple email accounts to the same email address. This is great if you have multiple people that need access to the same account but do not want everyone logging into the same mailbox. For example, if you have a support staff of three people and they all need to have access to the emails that are sent to email@example.com then a mail group is the perfect solution.
Before Plesk 10, there was always an option to create a mail group within the control panel. You could add internal or external email accounts easily by selecting them from a list or adding them in a frield, respectively. Although this process was easy to do, Plesk 10 removes the complication of Mail Groups and handles it all with mail forwarding. Instead of having a specific group with both internal and external accounts, you simply add whatever email addresses you would like the account to forward to within the Mail Forwarding tab.
Here is a quick demo video on how to setup and manage your Mail Forwarding and Mail Groups when using Plesk 10.