The post got closed as a duplicate Email: mail headers should have tags in them, possibly in reference to Customs email headers and/or subjects tags, but the latter is kind of general, and I’d like to talk about a specific problem we have:
We’re using tags as the most-logical replacement for dozens of mailing lists about various project sub-topics. We started using categories, but that became unwieldy — categories are heavyweight, don’t allow easy cross-posting, and for a whole other number of reasons, we think tags are a better match.[1]
However… there’s a problem. While it is possible to put %{optional_tags} in the template for email notifications, Gmail’s very limited filtering doesn’t do anything smart with this — and, even for the old-school mail user it’s kind of a pain to write a procmail rule that breaks down the subject line and parses it.
So, it’d be nice to have the tag somewhere else. For the procmail folks, a custom X-Tags header would do, but Gmail won’t care, so we need something else.
One idea would be to actually use tags as List-ID, but I’m not sure Gmail does the right thing with multiple List-ID tags mutliple List-ID fields aren’t permitted [2]. Another idea, which is a bit kludgy but I think would work: put tag@subdomain.example.org (and potentially also tag2@subdomain.example.org, tag3@subdomain.example.org, …) in the CC list. Google can filter on this, and most other systems also have reasonably-advanced features for dealing with CC as a multi-value field. And, we’d redirect any mail actually sent to these addresses to the void[3].
As a bonus, the CC approach could be used as a way to add tags on incoming mail (see Add tags by email). Again, kind of kludgy, but then, so is all email, in 2022.