warning: xxx.xxx.xxx.xxx: address not listed for hostname yyy.yyy.yyy
Postfix uses hostnames in its junk mail and mail relay controls. This means that in theory someone could be motivated to set up bogus DNS information, in order to get past your junk mail or mail relay controls. When Postfix looks up the SMTP client hostname for the SMTP client IP address, then Postfix also checks if the SMTP client IP address is listed under the SMTP client hostname.
If the SMTP client IP address is not listed under the SMTP client hostname, then Postfix concludes that the SMTP client hostname does not belong to the SMTP client IP address, and ignores the SMTP client hostname. A warning is logged, so that you can find out why an SMTP client is or is not stopped by your junk mail or mail relay checks.
You could contact the people who maintain the SMTP client's DNS records, and explain to them that each IP address needs one PTR record, and that this one PTR record needs a matching A record.
Some people read the RFCs such that one IP address can have multiple PTR records, but that makes PTR records even less useful than they already are. And in any case, having multiple names per IP address only worsens the problem of finding out the SMTP client hostname.