True; some of my mail providers *do* allow unsubscribing from some standard folders. Further, Interlink itself has some smarts along these lines. If you unsubscribe from a Drafts folder, regardless of its name, and then save a new message to it, Interlink automatically re-subscribes to the folder. That is perfectly reasonable.This is determined by the server you are connecting to. IMAP servers can have a set of folders configured that you cannot unsubscribe from.
Not sure of your meaning here. If that were true, then a fully-compliant client would get the list of whatever folders the server says must always be displayed, and grey-out the corresponding checkboxes, to prevent the UI from seeming to allow unsubscribing.[doing what you're asking] anyway would be breaking with the IMAP standard protocol.
Still, it is client's job to add usability wherever the standard (or the server's implementations) fails in that regard. For example, hiding folders the user wants to never see, regardless of the standard, is a great reason for open-source software to exist. Users can always re-subscribe, or clients can auto-re-subscribe whenever it makes sense to. If clients were required to display whatever folder names the server decides, then that would look to me like a defect in the standard.
Testing against GMail is no good, because they do a lot of non-standard IMAP chicanery that confuses Interlink, resulting in duplicate-looking special folders at different levels of the IMAP hierarchy, among other unexpected behaviors. Of course the client is going to use only one of the duplicates. Even futzing around with Account Settings -> Copies & Folders does not help. The Ggl server tries to act like a client, so the user experience can't tell whether the client's or the server's designated names will be used for operations on the special folders. Interlink cannot rename folders while they are designated as special folders in this dialog; this makes sense.
My reading of the implementations in all good IMAP clients I've used leads me to believe that even "standard" or "special" folders are supposed to be renamable and un-subscribable, unless the server's configuration prohibits that. (Read: the server's software never bothered become compliant by moving the special folder names from the source code to client-controllable config files.) If the IMAP standard actually prohibited these actions, then the UI controls for performing the actions would not appear in every full-featured program I've used (desktop clients, mobile clients, web-based clients).
Testing against a compliant server shows that Interlink supports assigning the "Drafts", "Sent", and "Trash" functionalities to folders having non-standard names, even when the server prohibits renaming or deleting the built-in folders. These built-in folders can properly be un-subscribed. An exception is - the server places a copy of the message in "Sent" regardless of what the client does. Thus, when any folder is designated as "When sending messages, automatically Place a copy in", the account ends up with two copies of the message in different folders. All this is a win for Interlink's compliance to the standard.
The server that is failing me is Dovecot. I can see the changes it makes to its per-account 'subscribe' files, and the special folders do not appear in there. Not sure if the ISP provides other IMAP servers.
Browsing the Thunderbird-class source code gives me a headache. Are you aware of any relevant discussions in the Moz development forums? Or, where can I find the most current relevant RFCs?