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Tuesday, October 13, 2009

exim: no IP address found for host

The problem:

Mails from a particular customer are frequently being rejected by our mail server (exim) because the hostname provided by the sending mail server does not resolve to the sending IP address. For example:

2009-10-13 18:29:31 no IP address found for host (during SMTP connection from [80.92.x.xx])
2009-10-13 18:29:31 H=( [80.92.xx.xx] F=<> rejected RCPT <>: host lookup failed (80.92.xx.xx does not match any IP address for

The solution:

In an ideal world the customer's IT department (or at least whoever is responsible for their IT affairs) would fix the problem at their end, as it would be in their own interest to do so. However, experience shows that it's hard to get hold of the right person or people, let alone explain what the problem is and what they can do about it (especially if several layers of outsourcing are involved).

Failing that, it would be convenient if our mail server software had some sort of facility for whitelisting particular IP addresses / hosts. Unfortunately however, mail server administration is not my speciality, and here experience shows that even if such an option exists, I can search for hours before definitively establishing its existence (or lack thereof) and implementing it in a way which doesn't screw with the rest of the mail server configuration.

Another possibility would be to switch off hostname validation entirely (if such an option exists), however this would open the floodgates to all the spam mail which is being rejected by this test (the current mainlog has 4965 such entries for the last 13 days).

Much simple, in the end, is to add an entry for the offending hostname in /etc/hosts, a matter of a few seconds and in the short term much less invasive than messing around with the mailserver configuration. It's useful to annotate what the entry is there for of course, and also bear in mind any issues which could arise through a particular hostname being "hardwired" to a particular IP address. The latter would be the case e.g. if another application on the same server needed to access other services under the same hostname (in one case I have had to add the primary domain name of a customer to /etc/hosts so we can receive their mails).

Posted at 1:25 PM |Comments (0)