In my lab, I am routing mail using DNS and noticed the below warning comes up often on the server. This is on Exchange 2016 CU17:
While the email does deliver it seems to stay in the queue for a few minutes and then it delivers. Other mail delivers instantly so I do not think this is a cause for concern.
On the other hand, it may also be the anti-virus as I didn’t see this when no AV was loaded.
Hope it helps.