On Tue, 18 Oct 2011 23:55:41 +0200
Karsten Bräckelmann <guent...@rudersport.de> wrote:
> The DNS TTL appears to be 12 hours, and a good share of mail
> (definitely true for ham, only partly for spam) is received from a
> rather limited number of distinct SMTP servers, only. With a local,
> caching DNS server the number of mail a system can handle per day
> before exceeding the free usage limit is *much* higher.
> number of mail != number of DNS lookups
That's true, though caching is much less effective than you may
suppose. In real-life measurements on real mail servers, I found a
very low cache hit rate for common DNS{B,W}Ls, on the order of only
25-50% hits.
Regards,
David.
