When people send us email, our system checks the incoming connection and performs some sanity checks.
This is done to make sure that when the connection comes in, the host that is making the connection really is who it claims to be. This involves a DNS check on the name <-> IP address mapping.
If this fails, or produces inconsistent results, then the connection is refused. A short message is returned to the sender informing them of what needs to be changed.
The reason it is refused is that people who send SPAM, and people who try to crack into sites usually do so in a covert way -- notably they often use faked IP addresses and unknown hostnames to try to hide who they are and where they are coming in from.
However, if the name <-> IP address translation is correct then the mail is accepted and everyone is happy.
If the name <-> IP address translation fails, then there is no way to verify that this machine really is what it claims to be. It could well be someone faking an IP address, or hijacking a domain to cover their nasty hacking activities.
Compare this with our domain:
$ nslookup haze.vcpc.univie.ac.at Name: haze.vcpc.univie.ac.at Address: 184.108.40.206
And the other way:
$ nslookup 220.127.116.11 Name: haze.vcpc.univie.ac.at Address: 18.104.22.168
So the name <-> IP address translation works both ways and produces consistent results.
This issue can probably be fixed by the network administrator (either at your site or at your internet provider) simply by making sure there is an entry for this machine in the reverse zone map of their DNS tables (a PTR record).
To compare again our domain
$ nslookup -type=ptr 22.214.171.124.in-addr.arpa. 126.96.36.199.in-addr.arpa name = haze.vcpc.univie.ac.at
There is lots of information on setting up DNS and Email at Stokely Consulting.