Mail Issues, it’s all a matter of time.

Posted on December 9th, 2007 by Luke Sheldrick.
Categories: IT / Tech.
Tags: , , .

9 12 2007

Last week, I started working on a new mail platform. I had in place, an Exchange 2003 box at home, and a backup mail server running Exim, based in Global Switch London.

I utilised Greylisting on both, however found the linux version ten times better. So set myself a task to setup a new front end, in front of my main Exchange box. Ruled out getting rid of it totally, as I personally prefer the features of Exchange (Push mail, calanders…etc).

So I setup a virtual machine (CentOS 4.4) on one of my virtual hosts, configured exim, with greylisting, and to relay all mail to my domains, to the internal IP of my Exchange box. I also wanted to setup a test Exchange 2008 box at the same time, so anything to my development domain, would go to a different internal IP.

After a few tweaks, magically it all worked… until I started to get NDR’s  stating that mail was being delayed to my domain. A few telnet tests later, discovered that if you connected from a host with no rDNS, my mail server took 30 seconds to time out doing a reverse DNS check. Many mail servers would have timed out by this time. A quick tweak of my exim config to lower the DNS time out, from 30 seconds to 5, cured the problem.

For anyone wanting to do this, edit the exim.conf lines

rfc1413_hosts = *
rfc1413_query_timeout = 30s

to

rfc1413_hosts = *
rfc1413_query_timeout = 5s

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