If DNS Servers are not available every mail will be rejected

[Mario Pruessner]

If DNS Servers are not responding that a domain cannot be translated then every mail which will be delivered will rejected.

That is not a good error handling. Please improve

[SH]

It is standard feature of spamfilter - if you trust the source you can easily skip filtering with relay/whitelist/authorization

If you have some kind of edge case, you can elaborate little more here