Reverse DNS Lookup

[Tim Smith]

Hello all,

A client of mine informed me that they did not receive some mail from 
someone in particular. When I examined the logs, I found the following 
warning:

*warning: hostname delivery.mailspamprotection.com does not resolve to 
address 198.143.161.12: Name or service not known*

I checked at MXToolbox and could see that 
delivery.mailspamprotection.com does indeed resolve to that IP address 
(along with a number of others!) and a PTR is in place also.

Can someone help me find out why Postfix believes this is an error? What 
mechanism does postfix use to resolve domains?

Thanks in advance,

Tim