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When I nslookup x.x.x.x my web server's IP address I get this error: Non-existent domain.

Must it return the top domain like mydomain.com or it is okay to return some.random.subdaomains.mydomain.com?

When I nslookup 144.122.145.140 (this is the IP of the email server. In the email I have received it said Received: from frigya.general.services.metu.edu.tr (frigya.general.services.metu.edu.tr. [144.122.145.140])) it returns frigya.general.services.metu.edu.tr However the emails are like [email protected]

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It doesn't have to return the top level domain. The forward and reverse DNS should match
i.e. nslookup x.x.x.x should return some.random.subdaomains.mydomain.com and nslookup some.random.subdaomains.mydomain.com should return x.x.x.x

This is done as sometimes the receiving mail server will perform this check in an attempt to detect if the mail is spam. You can see some more details and some actual examples here.

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