Hello and apologies for the delay on this! I checked in with development on this and they advised me that this error simply means we were unable to run rDNS checks against the IP of an incoming message. So, this isn't indicative of a problem necessarily at least, and you may be able to resolve that by adjusting your primary/secondary DNS server addresses in Settings>General. I am going to go ahead and escalate a task to have development review this area to make things more clear as well.