crates/common/src/auth/access_token.rs

@@ -553,6 +553,16 @@ impl AccessToken {
             || self.inner.access_to.iter().any(|a| a.account_id == account_id)
     }
 
+    /// inbuxa: MA-D0: in the account only because it is a group this token
+    /// belongs to. Such a member has the group's mailbox but may not share it
+    /// on: who is in a group is an administrator's decision, and a share
+    /// would let anyone in.
+    pub fn is_group_member_only(&self, account_id: u32) -> bool {
+        self.inner.account_id != account_id
+            && self.inner.member_of.contains(&account_id)
+            && !self.has_permission(Permission::Impersonate)
+    }
+
     pub fn is_account_id(&self, account_id: u32) -> bool {
         self.inner.account_id == account_id
     }

crates/imap/src/op/acl.rs

@@ -212,7 +212,7 @@ impl<T: SessionStream> Session<T> {
                 }
                 rights
             } else {
-                vec![
+                let mut rights = vec![
                     Rights::Read,
                     Rights::Lookup,
                     Rights::Insert,
@@ -223,8 +223,12 @@ impl<T: SessionStream> Session<T> {
                     Rights::CreateMailbox,
                     Rights::DeleteMailbox,
                     Rights::Post,
-                    Rights::Administer,
+                ];
-                ]
+                // inbuxa: MA-D0: a group's members don't share its mailboxes on.
+                if !access_token.is_group_member_only(mailbox_id.account_id) {
+                    rights.push(Rights::Administer);
+                }
+                rights
             };
 
             trc::event!(
@@ -266,10 +270,20 @@ impl<T: SessionStream> Session<T> {
 
         spawn_op!(data, {
             // Validate mailbox
-            let (mailbox_id, current_mailbox, _) = data
+            let (mailbox_id, current_mailbox, access_token) = data
                 .get_acl_mailbox(&arguments, true)
                 .await
                 .imap_ctx(&arguments.tag, trc::location!())?;
+
+            // inbuxa: MA-D0: a group's members don't share its mailboxes on.
+            if access_token.is_group_member_only(mailbox_id.account_id) {
+                return Err(trc::ImapEvent::Error
+                    .into_err()
+                    .details("This mailbox belongs to a group. Only an administrator can change who has it.")
+                    .code(ResponseCode::NoPerm)
+                    .id(arguments.tag.to_string()));
+            }
+
             let current_mailbox = current_mailbox
                 .into_deserialized::<email::mailbox::Mailbox>()
                 .imap_ctx(&arguments.tag, trc::location!())?;

crates/jmap/src/mailbox/get.rs

@@ -2,6 +2,8 @@
  * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <[email protected]>
  *
  * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ *
+ * Modified by Coffey Labs in 2026 for INBUXA.
  */
 
 use common::{Server, auth::AccessToken, sharing::EffectiveAcl};
@@ -14,6 +16,7 @@ use jmap_tools::{Map, Value};
 use std::future::Future;
 use store::ahash::AHashSet;
 use types::{acl::Acl, collection::Collection, keyword::Keyword, special_use::SpecialUse};
+use utils::map::bitmap::Bitmap;
 
 use crate::{api::acl::JmapRights, changes::state::JmapCacheState};
 
@@ -138,6 +141,11 @@ impl MailboxGet for Server {
                             JmapRights::rights::<Mailbox>(
                                 cached_mailbox.acls.as_slice().effective_acl(access_token),
                             )
+                        } else if access_token.is_group_member_only(account_id) {
+                            // inbuxa: MA-D0: everything but sharing it on.
+                            let mut acl = Bitmap::<Acl>::all();
+                            acl.remove(Acl::Share);
+                            JmapRights::rights::<Mailbox>(acl)
                         } else {
                             JmapRights::all_rights::<Mailbox>()
                         }

crates/jmap/src/mailbox/set.rs

@@ -613,6 +613,15 @@ impl MailboxSet for Server {
         // Refresh ACLs
         let current = update.map(|(_, current)| current);
         if has_acl_changes {
+            // inbuxa: MA-D0: a group's members don't share its mailboxes on.
+            if ctx.access_token.is_group_member_only(ctx.account_id) {
+                return Ok(Err(SetError::forbidden()
+                    .with_property(MailboxProperty::ShareWith)
+                    .with_description(
+                        "This mailbox belongs to a group. Only an administrator can change who has it.",
+                    )));
+            }
+
             if !changes.acls.is_empty()
                 && let Err(err) = self.acl_validate(ctx.account_id, &changes.acls).await
             {

tests/src/jmap/mail/acl.rs

@@ -2,6 +2,8 @@
  * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <[email protected]>
  *
  * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ *
+ * Modified by Coffey Labs in 2026 for INBUXA.
  */
 
 use crate::utils::server::TestServer;
@@ -713,6 +715,35 @@ pub async fn test(test: &TestServer) {
             .await,
     );
 
+    // inbuxa: MA-D0: a member can't share the group's mailbox on, and isn't
+    // told it may. Who is in a group is an administrator's decision.
+    assert_forbidden(
+        john_client
+            .set_default_account_id(sales.id_string())
+            .mailbox_update_acl(&inbox_id, bill.id_string(), [ACL::ReadItems])
+            .await,
+    );
+    assert!(
+        !john_client
+            .set_default_account_id(sales.id_string())
+            .mailbox_get(&inbox_id, [mailbox::Property::MyRights].into())
+            .await
+            .unwrap()
+            .unwrap()
+            .my_rights()
+            .unwrap()
+            .acl_list()
+            .contains(&ACL::Administer)
+    );
+    bill_client.refresh_session().await.unwrap();
+    assert!(bill_client.session().account(sales.id_string()).is_none());
+    assert_forbidden(
+        bill_client
+            .set_default_account_id(sales.id_string())
+            .email_get(&email_id, [Property::Subject].into())
+            .await,
+    );
+
     // Remove John from the sales group
     admin
         .registry_update_object(