Email forwarding is a service, which allows emails, sent to a specific mailbox to be also received in one or several other mailboxes, without having to be originally destined to them.
In order to enable forwarding for a mailbox, you can :
Right-click on the desired mailbox and choose the “Forward” option from the context menu;
Select the mailbox and click on the “Forwarding” button, above the table with the email accounts;
Press the icon in the Forwarding column, next to the desired mailbox;
All actions will result in a new popup, from where you can specify the mailbox you wish to forward emails to, and should a copy of the forwarded email be left in the original mailbox.
Once you are done, click on the “Add” button to add the new email filter.
NOTE: email forwarding cannot be enabled for a catchall mailbox. You will have to disable the catch-all service in order to active email forwarding for the desired mailbox.