-Limitations in the duplicate message handling are the main reason for not
-indexing the `List-Id:` header. If you receive the same message via both the
-list and directly (by way of `To:` or `Cc:`), only the first message encountered
-will be indexed. Only the message received via the list will have `List-Id:`,
-but you would expect a `List-Id:` search to find the message, regardless of the
-order in which the duplicates were received. This is a more general problem than
-just `List-Id:`, and once this has been resolved, adding `List-Id:` indexing is
-trivial.