faq: how do I search for the list-id: header

diff --git a/faq.mdwn b/faq.mdwn
index bb0efaa90c2b458ca5e6ec9ea1486e52d7da072a..17412884ab23f09a0a73e5a31b3630d9bc101436 100644 (file)
--- a/faq.mdwn
+++ b/faq.mdwn
@@ -53,3 +53,18 @@ searches. Punctuation is mostly ignored.
 
 The boolean prefix searches (see Boolean and Probabilistic Prefixes in the man
 page), such as tag: or path: searches, need an exact match.
+
+## How do I search for the `List-Id:` header?
+
+Currently there is no way to search for `List-Id:`. As a workaround, try using a
+`to:` prefix search (which covers both `To:` and `Cc:` headers) on the mailing
+list address.
+
+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.