I just committed support for NAMESPACE in Tinymail. This means that now you can show your boss’s shared folders in Tinymail based E-mail clients.
The “Shared” and “Other” namespaces are often used to put large lists in like NNTP newsgroups, so I only support the list LSUB returns for those two namespaces. LSUB gives you only the mailboxes that you are subscribed to. For the “Personal” namespace, I get both the LIST and the LSUB or in case you have LIST-EXTENDED I ask for LIST (SUBSCRIBED) and filter-out the \NonExistent ones, and flag the \Subscribed ones.
The better way to use “Shared” and “Other” is probably by not recursively asking for the entire list. In stead, when the list is unfolded you ask for the flat list of children under the level of the list. This of course means that you can’t know whether or not the UI must show an unfold arrow. For that you have the \HasChildren flag in IMAP. I’m not yet using this, though. More work for me, yippee!
Once this is done, you could make a UI to help to user subscribe to folders in accounts that have very large amounts of folders.