better blocking at MTA level (off-topic)

[Dhawal Doshy]

Jason Ede wrote:
> The problem with greylisting as we found when we enabled it was that 
> some MTA's treat a temporary reject code (450) as a permanent reject 
> code (550) and our customer was complaining that they weren't getting 
> email. Whilst we could exempt every MTA they flag up to us from 
> greylisting checking there is no way to prevent email from an unknown 
> source getting blocked and not getting through to them. Until we can 
> resolve the problem without telling our customer to tell anyone emailing 
> them with problems to get their MTA fixed it unfortunately has to be 
> off... It did vastly reduce the load on our server whilst greylisting 
> was active though.

Change 450 to 451 to reduce false positives.. exchange responds better 
to a 451 (as compared to 450). See 
ftp://ftp.rfc-editor.org/in-notes/rfc2821.txt

       450 Requested mail action not taken: mailbox unavailable
          (e.g., mailbox busy)
       451 Requested action aborted: error in processing