I am doing this as part of bug 474711 and logging a separate bug so the feature shows up in the fixed list. Although the code defined a mailing list attribute, nothing actually populated it. Now we will. The "mailingLists" attribute (as bound; canonical name is "mailing-list") is non-singular, so other extensions could probably capture cross-posts for (tunneled) newsgroup messages or other pseudo-mailing-list things. We are using the List-Post header and do nothing clever involving messages that the user sent to a list let alone multiple lists. We are definitely not clever enough to fuse together a single cross-posted message that the user receives multiple times from the different mailing lists. Over-achievers are more than welcome to resolve this.
Bug 474711 has landed with this fix.