Lucio Crusca writes:
Hello everybody,mxtoolbox says that my SMTP is slow: http://mxtoolbox.com/domain/sulweb.org/ It's a Debian GNU/Linux 8.1 64bit kvm guest, 6 vCPUs, 2GB of RAM and a load average that is almost always below 0.5. iptables blocks OUTPUT to 113/tcp with REJECT. "courier show all" shows only one domain (that doesn't go away with "courier clear all" but that's another problem, because the blacklisted domain has nothing to do with mxtoolbox). There aren't any DNS block lists configured. The configured DNS is a local Bind9 with recursion. What is taking nearly 13 seconds to complete the transactions that mxtoolbox expects to complete in less than 5 seconds?
Given what you posted, the only possibilities are slow DNS lookups. Each incoming IP address is resolved, forwards and backwards, via DNS. Additional, the domain of the sender's address is also looked up in DNS.
If you have poor network connectivity, your DNS lookups could take several seconds, each, to complete.
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