Question : DNS causing infinite mail loop

I'm almost certain this is a DNS problem, not one with the mailserver.

I've got a MailEnable mailserver running on my fasthosts dedicated server, and a very weird thing seems to be happening.

99% of the time, mail is fine and there isn't a problem. But every so often, the mailserver sends to itself infinitely:

I looked in the logs and can see in the message headers that the SMTP connector sends the message to itself, which it then receives, begins to send again, resolves the target domain to itself (this is the part going wrong), sends it off to itself, and this repeats until it detects the loop and quarantines the mail.

The important part here is why the DNS resolves back to my own server for addresses that should be external !

By running nslookup and doing a few checks, the first thing I noticed was if i put in 'google.com', i get 'google.com.gloucester.mybusiness.com'. Now, is that just that i'm leaving the final '.' ? - I've not used nslookup a lot.

It's probably relevant to note that the 'full computer name' of the server is 'server1.gloucester.mybusiness.com'. This might be where it is coming from (i.e auto DNS suffix).

It's also relevant to note that the server I am doing the nslookup from is not my own DNS server, but one somewhere in the fasthosts network, and the one which is configured as the DNS for the servers network connection.

I appreciate there's probably a lot more detail needed for an answer to this one - but am hoping some suggestions on where to start looking would be put forward.

Thanks in advance.

Answer : DNS causing infinite mail loop

You have at least two problems:
If a domain foo.bar does not resolve, the dns client tries to append DNS suffixes and tries again. This is normal behaviour in order to make life easier with local hostnames. However, in your case it seems that even some prominent domains do not always resolve fast enough, thus causing that second try. You don't need to remove the DNS suffix itself, but have a look at a checkbox (I don't knwo how it is labelled in English) on the DNS configuration tab of network properties that decides whether or not to use the suffixes for resolution.
Second, your internal nameserver seems to have a wildcard record, which causes anything.gloucester.mybusiness.com to resolve to your IP. Such a wildcard can be a good idea to support e.g. mass virtual webhosting; but in your case it seems to co-cause the problem.
There should at least be no wildcard record for a domain used as DNS suffix. Check carefully if this wildcard is necessary and if not - remove it after adding the few hostnames it is really needed for.