extensions to local parts of mail addresses

I wonder why nobody is coming up with the idea of a
more generalized way of extending mail addresses:

- It seems that for some purposes it is useful (on
  a global base) to associate additional information
  to an e-mail address. The example given is to
  add 'a subaddress'. 

- One might think about a general mechanism like for
  example:

  <"localpart;key=value;key=value"@domain>

  in order to associate different things to an address. 

- Examples: 

  To: "someone;reply=desired;receipttype=yes/none/negative"e(_at_)foo


- Of course this causes interoperability problems at several
  levels. (details left out as an exercise.) These need
  to be addressed, yes, that's part of the work to be done.

Peter