Automatically creating user accounts from exim

[David <wizzardx@gmail.com>]

Okay, this is kind of a weird question, but it came up at work.

I'm a complete exim newbie (I've never configured it before, beyond
'dpkg-reconfigure exim4-config'), but a project came up where the
manager wants to use exim in a weird way. Basically, this needs to
happen:

1) Exim receives a mail, from a trusted IP address

2) If the mail is to a non-existant user account, then create the
system account,  deliver the mail to the new account's mail file

3) And always, after delivering a mail (for new or existing users):
Call an external script, so that our custom logic can see the new
mails immediately after they appear, and do some further handling.

I've tried looking through the reference docs from the
"exim4-doc-html" package, but they're confusing to an exim beginner.

Any tips?

Thanks in advance,

David.