Email/query: inMailbox filter ignored when combined with keyword filters

[DasBen]

Disclosure: Yes the Issue was written with the help of AI, but its so much more structured 😉

Environment

• Stalwart version: v0.11.x (Docker image)
• Protocol: JMAP (urn:ietf:params:jmap:mail)
• Client: Custom JMAP client (Node.js, raw HTTP requests)

Description

When using Email/query with a FilterCondition that combines inMailbox with keyword filters (notKeyword, hasKeyword), the inMailbox condition appears to be silently ignored. The query returns emails from all mailboxes instead of only the specified one.

Per RFC 8621 §4.4.1, all properties in a single FilterCondition object should be implicitly ANDed together.

Reproduction

Setup: An account with emails in multiple mailboxes — Inbox has 4 emails (all read), Junk has 6 emails (2 unread).

1. inMailbox only — works correctly

["Email/query", {
  "accountId": "...",
  "filter": { "inMailbox": "<inboxId>" },
  "sort": [{ "property": "receivedAt", "isAscending": false }],
  "limit": 50
}, "q1"]

Result: 4 email IDs returned — all belong to Inbox. ✅

2. inMailbox + notKeyword — inMailbox ignored

["Email/query", {
  "accountId": "...",
  "filter": { "inMailbox": "<inboxId>", "notKeyword": "$seen" },
  "sort": [{ "property": "receivedAt", "isAscending": false }],
  "limit": 50
}, "q1"]

Result: 2 email IDs returned, but fetching them via Email/get reveals they belong to Junk Mail, not Inbox. ❌

3. No folder filter + notKeyword — same result as test 2

["Email/query", {
  "accountId": "...",
  "filter": { "notKeyword": "$seen" },
  "sort": [{ "property": "receivedAt", "isAscending": false }],
  "limit": 50
}, "q1"]

Result: Same 2 email IDs as test 2 — confirming that inMailbox was ignored in test 2.

4. FilterOperator AND — returns 0 results

["Email/query", {
  "accountId": "...",
  "filter": {
    "operator": "AND",
    "conditions": [
      { "inMailbox": "<inboxId>" },
      { "notKeyword": "$seen" }
    ]
  },
  "sort": [{ "property": "receivedAt", "isAscending": false }],
  "limit": 50
}, "q1"]

Result: 0 email IDs returned, even though test 1 confirms there are emails in Inbox. ❌

Expected Behavior

• Test 2 should return only emails that are in Inbox AND do not have the $seen keyword (i.e., unread emails in Inbox only).
• Test 4 should produce the same result as test 2 since FilterOperator AND is the explicit form of combining conditions.

Actual Behavior

• Test 2: inMailbox is ignored — query matches across all mailboxes, returning unread emails from Junk instead of Inbox.
• Test 4: FilterOperator AND returns 0 results instead of the correct intersection.

The same behavior occurs with hasKeyword (e.g. filtering for $flagged within a specific mailbox).

Impact

Any JMAP client filtering emails by keyword within a specific mailbox (e.g. "show unread in Inbox", "show starred in Sent") gets incorrect results. This is a very common operation for email clients.

Workaround

Query with inMailbox only, then filter by keywords client-side using the keywords property on the returned Email objects.