To drive this more Off-Topic than it already is... > Mailing list software for decades inserts unique headers for filtering > purpose. "List-Id" is AFAICT the most common one, which is also used > by GNU mailman. It has been specified in 2001 in RFC2919
Even non-Mailinglist software, e.g. GitHub, EBay, Deutsche Bahn, are supplying custom List-Id headers for separate topics. In case of GitHub one can use List-Id headers to automatically separate mails for different repositories without looking at the content or Subject. Cheers Andrej
