I know this is a little off-topic, but in some sense, the more general requirement is how do you integrate/link your emails into your GTD system. Tagging is one approach. Another is to link to your emails via URLs, or some other linking system.

I wish there was an easy way to have a URL to uniquely identify an email message. I use TiddlyWiki to maintain my GTD data, and occassionally, it is nice to refer to a specific email message by a URL.

At the moment, all my business emails are on an IMAP server, so I have installed Squirrel Mail, which gives me the unique URL -> email reference, so in TiddlyWiki, I can click on a link to see the associated email of interest, that may be relevant for a project or task.

However, other systems such as gmail, from what I can tell, don’t give this to you easily, if at all. And as for other email clients… I suspect it won’t be easy.