Retry when the DNS server is not responding
It's more a bug than a feature, but you CS referred me to here.
I ran into a problem when Mailgun doesn't route emails to my end, since "the DNS server didn't respond at that time"
I really tried to understand why you just don't send the request again and try to reach the server after a few minutes?
The DNS server should always be available, so it just makes sense to send the request again and try to route the email.

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Raymond Cheung commented
I am facing the same "Routes" issue while Mailgun claimed that DNS name is not able to be resolved.
Therefore a Routes Retries are very important to make sure the emails are being route to the destination URL.
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Raymond Cheung
OptiTable Technology Limited