[exim] Quota understanding problem

Hi

I have an installations of exim4 with courier-imap which needs to impose
virtual quota.
No system users are involved, so no file system quotas are appliable.

No I can set "quota", "quota_warn_message" and "quota_warn_treshold" in my
local delivery transport.
(BTW, quota_is_inclusive is still default "yes")

But it seems not to work correctly:
1. If a mail arrives, which would exceed the quota, it is NOT delivered.
(correct)
2. If a mail arrives, which would exceed the threshold, it is delivered and
a warning is delivered. (correct)
3. If a mail arrives, which would exceed the quota and the threshold at the
same time, it is NOT delivered (correct) and NO WARNING is delivered.
(INCORRECT?)

Moreover, I have a problem with the design of quota overall:
If the mailbox grows because of mail which is stored by courier-imap (for
sent-items i.e.), exim does not send a warning.
This is correct behaviour regarding to the exim specification, however this
is not what I want.

How do you guys handle quotas?

Can someone tell me a "somehow good" way to manage quotas when using exim4
in conjunction with courier-imap?

Regards,
Steffen