[exim] Address rewriting for spam control

In the past, I have found it useful to fabricate addresses for my own
domain when signing up for services that required valid email
addresses. For example, if lamesite.com requires me to list an
address, I'll give them myname-lamesite.com@???, knowing that
all mail to that domain winds up in my single inbox. This scheme has
worked rather well to show what domains have distributed my address.

My problem now is that I need to host several distinct user mailboxes
on the domain. I want to be able to process incoming messages and
deliver them to the correct party without configuring exim ahead of
time with those addresses. If we presume for a moment that I use the
hyphen as a delimiter, the following addresses

user1-losers.com@???
user1-morons.com@???
user1-idiots.com@???

should all be delivered into user1@???. Bonus point will be
awarded for adding headers beased on the second half of the
destination in the format-

x-betray: losers.com

Based on my casual understanding, I should be able to the first
criteria with rewriting techniques. I'm not so sure about the second
criteria.

I invite your educated opinions...