Harald Meland wrote:

... and before anyone gets religious on me here, I'll rush to explain that I _do_ see the harm of inserting "faulty" References: headers -- any In-Reply-To: candidate should at the very least be in the form of a valid message ID.

I think there was a slightly similar discussion on the (ding) Gnus mailing list a while back -- it was spurred by the fact that Gnus' threading sometimes broke down, as some mailers put several message-id-look-a-like-tags (e.g. mail addresses in angle brackets) in their In-Reply-To: header.

Threading is already broken on the newsreader side if References: doesn't exist. I'd say that as long as it has the correct form, pass it on. (I'm also making the possible faulty assumption that the Gnus problem manifested in the context of reading a mailing list rather than a newsgroup.)