Greylisting with reject_unverified_sender negative cache

[Mick]

Good morning,

I have found 'reject_unverified_sender' superb at reducing the number of 
SPAM messages getting though.  I've set up a whitelist for those few 
trusted senders or domains where their dopey mail servers' don't comply. 
I do have a minor problem with mail servers that do comply, but apply 
greylisting.

Originally I omitted 'address_verify_negative_cache =no' from main.cf. 
This defaults to 'yes' and sender verification failures were cached 
saving a constantly chattering probe relay. Unfortunately it would 
appear that this method also caches temporary errors too (those also 
being a fail), so when I receive a 471 as part of a greylisting policy, 
that message won't be delivered.  Postfix will reject when remote server 
re-attempts to deliver relying on its cache from the first attempt 
rather than sending another dummy message. I have now set the negative 
cache to 'no'  meaning a retry for every incoming message that hasn't 
passed address verification. It is either that or adding all domain that 
use greylisting to the whitelist.

Does anyone know if there's a way to exempt / prevent 471 (or other 
temporary reject codes) from being cached?


Thanks,

Mick.