Admin-Managed Shared Mailbox Access in Nextcloud Mail

Is your feature request related to a problem? Please describe.

In organizational environments (e.g. associations, SMEs, healthcare facilities, schools), email communication is often based on shared mailboxes such as info@, support@, or office@.

Currently, shared mailboxes in Nextcloud Mail must be manually configured per user and per client.
There is no central administrative interface to:

• Define shared mailboxes
• Assign users to mailboxes
• Control access and sending permissions centrally

This results in:

• High administrative overhead
• Inconsistent configurations
• Increased error rates, especially during onboarding/offboarding

Describe the solution you'd like

Extend Nextcloud Mail with an admin-level mailbox management interface that allows administrators to:

• Centrally define shared mailboxes (email address / IMAP account)
• Assign users to those mailboxes
• Define per-user permission levels:
  • 🔍 Read-only
  • ✉️ Send
  • 🔧 Full access
• Control mailbox visibility and sending rights independently from a user’s personal email setup

This feature should integrate into the existing Nextcloud account and permission model.

Describe alternatives you've considered

• Centralized management of shared mailboxes
• Reduced setup and support effort
• Clear role-based email access
• Improved suitability for multi-user organizations

Additional context

• Group-based mailbox assignment
• Audit log for mailbox access
• API support for automated provisioning

No implementation files or tests are identified in the issue. Start by mapping the existing Nextcloud account and permission model to the proposed admin mailbox interface, including mailbox assignment, read/send/full-access rights, visibility, and API support. Done should be defined as an agreed end-to-end design and implementation scope.