450 4.7.1 Client host rejected: cannot find your reverse hostname, [] - Error Message

I have a user that is getting this message while forwarding an email to someone who they normally can send mail too:

450 4.7.1 Client host rejected: cannot find your reverse hostname, [xxx.xxx.xxx.xxx]

About a week and a half ago we changed our ISP from 1 AT&T T1 line to a Verizon T1 + 3.0 Mbps DSL combination connection. I'm curious if this would have anything to do with it, but it seems as though it's this one user......

I used Sam Spade and did a "Whois" lookup on the IP address that the world sees us as and it comes back with this....
     No match for "xxx.xxx.xxx.xxx"

Anyone know what the problem is here and what I can do to resolve it?
fnbtechPC / Network TechAsked:
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agriesserConnect With a Mentor Commented:
You need to ask your new provider to create a reverse lookup entry (PTR record) for your maildomain name to your new public IP address. Usually, if you host your own mailserver, it is called something like "mail.mydomain.com" and points to the public IP you posted above. Make sure your provider creates the above mentioned PTR record so that the IP address does resolve back to the domain name too. You cannot do that in your domain administration panel where you host your domains, since the IP address belongs to your provider, so only he can do that.
fnbtechPC / Network TechAuthor Commented:
Ok, I'll contact them. Thanks for the quick response. Once it's all finished, I'll gladly apply the points.
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