MX Record lookup and check

With dnspython module (not built-in, you must pip install it):

>>> import dns.resolver
>>> domain = 'hotmail.com'
>>> for x in dns.resolver.resolve(domain, 'MX'):
...     print(x.to_text())
...
5 mx3.hotmail.com.
5 mx4.hotmail.com.
5 mx1.hotmail.com.
5 mx2.hotmail.com.

Take a look at dnspython, a module that should do the lookups for you just fine without needing to resort to system calls.