[Dovecot] Warning recipient when message delivery fails due to over-quota

[Ben Johnson]

I'm working to configure automated quota notifications in Dovecot and am
wondering if it is possible to send a warning message to a user when
message delivery fails because the user is over-quota.

I already have the following directives configured:

  quota_warning = storage=95%% quota-warning 95 %u %d
  quota_warning2 = storage=80%% quota-warning 80 %u %d
  quota_warning3 = -storage=100%% quota-below below %u %d

These seem to function as expected, but the problem I'm facing is that
when a message is large enough to take the user's quota from, say, 84%
to over 100%, the message is rejected (as expected), but the user is
never warned that a message failed to be delivered because it would have
put him over 100% usage.

I suppose that I'm looking for a trigger that is tripped when delivering
the message *would* put the user over-quota (as opposed to *does in
fact* put the user over).

The sender receives an automated rejection message already. I'm
wondering if the recipient can be warned at the same time. (I'm not
concerned about further increasing the user's quota consumption with the
warning message; I would set noenforcing to 1.)

Is this possible? Any help is much appreciated. Thanks in advance.

-Ben