If you want to access a mail folder shared by some other user, and this user is not the Administrator or the manager of your community, you need to create a link to the shared mail folder. This is to give you the choice of which mail folders you want to access and which you don't.
Imagine that user joe has shared a mail folder with the name project x with you. To link to the shared mail folder:
click on Mail on the DeskNow menu
click on New link
input the username of the person that is sharing the mail folder (in this case, input joe) and click Next
you will be shown a list of all the shares that are available to you
select the share that you want to access (in this case, select project x)
input a name for this link - or leave the default name