Misconfigured or compromised

Why am I seeing a note about "Misconfigured Server (high probability of compromised host)"  in Threat/Unknown? (either directly in the Detail Viewer or via an email)

If you received a notice of this in email, this can come up because some entries in the Threat/Unknown Detail Viewer category were recently (around 2022-04-07) placed into a grouping. Previously the nxdomain rows were ungrouped, but you probably already had them occasionally. The new grouping just triggered the source notification.

The listing title itself indicates the status(either Threat or Unknown) of sending hosts that 

a) have unconfigured or misconfigured PTRs, and

b) have been reported as a source or email using your domain in the From header