Just checked them out. Looks to me like they don't offer 'real' shared mailboxes (where every user authorized for the mailbox has the same view of the mailbox), just 'distribution lists' (in essence, forwarders).
Consider the following scenario: Alice and Bob jointly manage a mailbox sales@acme.co. An email from a customer arrives and Alice replies to it.
With a real shared mailbox, Bob should be able to see the e-mail Alice sent, as they both have the same view on the shared mailbox. IIUC, with a simple distribution list this is not the case (the sent e-mail just resides in Alice's personal mailbox).
