For those of you who use Mail.app instead of the other bloated counterparts but wish for the ability to tag your mail beyond smart folders, Indev has a very productive plugin for Mail.app called MailTags as a donationware download.
In addition to tagging, it integrates nicely with iCal for project support, due dates and notes. These features not only work with messages in your inbox, but also with your outgoing mail. Recipients will not see your tagged information, but it will be accessible to you. Spotlight searching is available to all Mailtags data; tags, notes, projects, etc. Smart mailboxes can be created based on any Mailtag criteria.
One of the nicest features I see in this is that you can create Mailtags based on rules. For example, you can set a rule to create a due date 1 day ahead for all mail coming from firstname.lastname@example.org.
If you are a GTD fan, this should fit in nicely with your workflow. It’s well worth the download.
I will have to test this with MailSteward to see how it works or if it will save the data with my mail to the MailSteward database. I haven’t seen any information on it’s integration yet.