I have a really annoying intermittent problem where attempts to mail to valid sites are rejected with unknown mail domain. This only happens a few times a day and there is no particular pattern to the sites it rejects. If I try to lookup the same domain a few minutes later then it is OK.

This is almost certainly a problem with the DNS resolver or the the domain's name servers.

1. Have you linked Exim against the newest DNS resolver library that s with Bind? If you are using SunOS4 that may be your problem, as resolver that comes with that OS is known to be buggy and to give rmittent false negatives.

2. Effects like this are sometimes seen if a domain's name servers get out of step with each other.

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