ML mail writes:

My fault here, I should have precised that the DNS query it tries to resolve is simply the DNS name of my mailbox server (mailboxserver.domain.com). So domain.com is hosted locally on DNS servers on that very same network as the dovecot servers are located. Furthermore all the resolvers I use in /etc/resolv.conf are also local DNS resolvers on that same LAN. All the DNS servers (authoritative and recursive) are using PowerDNS. That's why I was really puzzled to see that it can take up to 1.5 seconds to query my own local DNS servers which are in no way overloaded.

That really doesn't change my answer, but since you control the entire DNS chain of query and response, you can further debug it at the server end via logs or network tracing. And you should still run a comprehesive DNS trace (e.g. http://dns.squish.net/) since a misconfiguration could still cause your DNS resolver to go on wild goose chases. In either case, it's probably not Dovecot related.

If you want to triage it, just enter static entries into your proxy's /etc/hosts file. Or use bare IPs in your configurations.

Joseph Tam jtam.home@gmail.com