Dns sax vdyo
Just use nslookup again, this time ask for the name you've got in the first query.
You might notice that the returned IP address(es) don't match the IP address that has been there at the beginning.
In the days of the Internet predating the World Wide Web, when large FTP servers were the biggest resource providers in the Internet, it was already quite common to reject clients that did not have a correctly maintained DNS reverse pointer (since system administrators usually logged all the people visiting them, so in case some problem turned up later, they at least knew whom to blame).
With the massive growth of the Internet, the average knowledge among many ISPs and other network administrators, as well as their discipline against perceived "minor things" heavily decreased, so right now, DNS setup failures can be seen all over the Internet.
Either option will give you access to all of the hosting capabilities of App Service: In many cases, upgrading will be as simple as switching to the new Mobile Apps server SDK and republishing your code onto a new Mobile App instance.
There are, however some scenarios which will require some additional configuration, such as advanced authentication scenarios and working with scheduled jobs.
In case you are a dialup user, you should use your ISP's mail gateway anyway, and hopefully even the sloppiest ISP will have a working PTR record for at least this machine.
There's one common error that might happen if the person maintaining the reverse zone file forgot to end the record with a dot. This is just a simple and obvious mistake which should be easy to fix for the person in power.
So lazy or sloppy network administrators might forget about reverse registration (while they usually do the forward registration thing, since it's obvious you quite often do want to have a name for your address).
Also, since both entries are maintained separately, sometimes perhaps even by different administrators, this procedure is error-prone: the name yielded at a reverse lookup might not point to the same IP address when looking up its A RR again. There always has been the requirement that forward and reverse data are maintained consistently in DNS.
Thus it has been statistically proven that a lot of spam can be avoided by not accepting emails from sites that do not have a valid DNS reverse setup.
(This is only one of our measures to avoid accepting spam in the first place.) If you did not want to send us (or any of our users or sites) a spam email, but have been rejected due to a broken reverse DNS setup, first watch out what actually is broken.