Keeping this simple; if we have quotas deployed (Dovecot/Maildir) and
using LMTP that means that Exim has already accepted the message and is
using LMTP transport to hand off for the final stage of the delivery.
Dovecot can be configured to send back a 5xx or 4xx failure if the user
is over quota. Lets look at them one at a time... what's the best action?
4xx - Mail would queue and retry on the MX/MTA, probably almost
indefinitely, especially for inactive mailboxes. Ultimately these emails
would just sit around and eventually an NDR would be generated. Probably
not a good idea unless the queue/retry was something short to avoid some
temporary disk space shortage. (maybe an hour or two)
5xx - Permanent failure, Exim would have to generate an NDR.
I guess in both cases am I correct in that Exim will have to generate
NDR's back out to the senders? My concern is with blow back and
excessive noise trying to go to spam sources, etc.
What are others doing? Would just a really short retry/lifetime to send
the NDR be the best course of action? ie: Try to send the NDR X times
within a period of time and then just ultimately give up. This way it
doesn't clog the queues up with NDR's that are ultimately not going to
be delivered...
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