Could someone please extend mail?

I waste a lot of time archiving mail by project. So the question is: why can't we just extend mail fields?

Just imagine:

• Fields would be urn based, and a set of basic fields would define:
• Mail applications could create templates extending fields;
• Fields would be sent on reply;
• When filling fields, mail applications could select from a used list of values.

If we could extend mail, a "Project" field would probably the first one we's use. a

PS: here's how I use the "Project" field today: I embed it on the subject field, something like "[Project A] Kick-off meeting".