Email tag?

Just organising my mailboxes with filters and rules, now what if emails have tags? Just like some people try to do that in the subject line of the email, such as [PROJECT-A] call for meeting. That actually helps me think of what to deal with the email quickly. Of course, there can't only be pros about one thing. I am so certain that spammer will abuse the tag.

More research on this topic, and I'll come back to discuss later.