[exim] Rate-limiting outgoing mail processing per-domain

We run exim relaying mail for our customers, as a virtual ISP. These
customers generally use SMTP AUTH, although some do use squirrelmail.

I know that exim can control the incoming rate quite easily, but we would like
to rate limit the *outgoing* RCPT TO: rate per authentication source when
processing the queue, without limiting the incoming mail rate.

If a customer exceeds their set limit, we can then notice that the queue is
building up and ...
* do nothing (we don't care)
* allow a higher rate (we love and trust them, and they pay for it)
* enforce incoming rate limit for that customer (we think they are being
stupid)
* kill their account (they are being evil)

How can exim be configured to rate-limit outgoing mail?

&:-)