[Dovecot] Deferring, instead of rejecting, messages when at quota

[Mike Brudenell]

Greetings -

I'm having trouble working out where to alter this behaviour (so  
please forgive me if it turns out to be an Exim thing rather than  
Dovecot!)...

My test service is using Exim as the MTA and Dovecot's "deliver" for  
the LDA.  We are using Maildir mailboxes with filestore quotas.

When I send a large test message to myself that cannot be delivered  
because I don't have sufficient available quota remaining an error  
message is immediately returned to the sender and the incoming  
message discarded.  The error message says this:

     Your message was automatically rejected by Dovecot Mail Delivery  
Agent.

     The following reason was given:
     Not enough disk space

Instead of Exim rejecting the incoming message and generating the  
error response because of deliver's failure to, umm, deliver I'd like  
the message to be retained in Exim's queue so that it can be retried  
periodically.

With our old/live system (Sendmail + UW IMAP's "tmail" LDA) I edited  
the tmail source code to return a different exit code: one that  
caused Sendmail to defer (re-queue) the message rather than reject it.

How do I achieve this effect with Exim + deliver please?
Is it a case of:

   * Setting something in Dovecot's configuration file (I can't spot  
what!);

   * Changing a return code in deliver's source (if so, whereabouts  
and to what?);

   * Or is there an Exim directive I need to add to the transport  
definition in
     its configuration file?

With thanks,
Mike B-)

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