courier: DNS temporary errors should cause temporary delivery failures
In the SMTP courier, when the MX lookup fails with a DNS temporary error, we should propagate that up and cause delivery to fail with a temporary error too. That way the delivery can be attempted later by the queue. However, the code today doesn't make the distinction and will treat any temporary DNS error as a permanent delivery failure, which is a bug. The bug was found by ludusrusso and reported in https://github.com/albertito/chasquid/issues/4 This patch fixes the bug by propagating the error properly.
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