unrouteable mail domain - not the usual suspects

Hi,

I've seen all the threads where it's a problem with not letting nobody send remotely, or outgoing SMTP limits. Well this isn't either of those, this one is a little different. Client sends email to an address someone@example.org - example.org has no A record, but it does have an MX record - Which is perfectly valid, and Exim will try and deliver, however, if there is a problem with the delivery, things get a bit tricky:

451 <someone@example.org>... Requested mail action not taken: mailbox unavailable

So exim defers the delivery, which is fine. What it does next, is the problem, when it retries defered email to the domain with only an MX, it fails it with:

unrouteable mail domain "example.org"

Although the mail domain is perfectly routable as it has a valid MX record which can be looked up.

Thanks,