"shared%user" login capability -> some email clients

Feature Ideas
1

Some email clients and use cases, it’s beter to have a “login” to the shared/group account, than just the shared folders

reference: Group mailboxes "direct" login option?

The login syntax already exists: shared%user is the composite shape you are describing, and the session it produces is the one you want. A group access token carries the group’s own mailboxes with empty accessTo and memberOf, so the session sees that account and nothing beyond it, with its own Trash, Junk and Archive. So no, it would not fight the group design; the session model already does the right thing.

The obstacle is on the authorization side. impersonate is all or nothing: the check is that the caller holds the permission, never which targets they may reach. Giving it to ordinary users so they can open one shared mailbox would hand them every mailbox on the server. The default Group role also grants the IMAP and JMAP families without authenticate, which is what “groups cannot log in” amounts to in practice.

So it is implementable, and what it needs is a per-target delegation grant, not a wider impersonate. You can file this under Feature Ideas.