Nice to come across this topic. I’ve been dreaming of writing my own email client exactly for this reason but haven’t gotten to it yet (surprise). In addition to the comments made already, I’d go even further and allow a hierarchy of tags, mandatory tags etc (so that if you work with others your “structured” email archive is built-up in a similar way). With a mandatory tag I mean something extra to the “Subject” line that is not free text but uses a structured list - that is created on the fly as needed. Obviously some more thoughts about collaboration with groups need to go into this…

My question: has there been any progress lately re this that you are aware of?

In case it helps, Thundervbird stores emails in raw format — just plain text, exactly as received. The .msf files that it creates store information like sorting and so on. I don’t know the format of those — you’ll have to look at the source. Good luck.

One way to implement this is to store all emails and tags in a database (in the obvious way), and display each tag as a folder. You could cache the headers of the emails in each folder, as Thunderbird already does. Storing the email in a database would also improve searching, since you could have a full-text index.

All this might actually be fairly easy to implement, at least for POP3 mail. A feature like this could capture the zeitgeist and propel Thunderbird to Firefox’s dizzy heights. Well, I would love it anyway.

I made a similar observation to yours here.

http://mentalcompost.blogspot.com/2005/06/tag-friendly-mail-client.html