Sorting email address according to occurrences of each domain
I suggest you get a linter such as Prospector or flake8, this can tell if your code is un-Pythonic. Some people prefer hinters like black.

Your code doesn't conform to PEP 8 which is the standard when it comes to Python. Your comprehension is hard to read because it doesn't conform to best practices.
I'd recommend moving your code into a main function and use an if __name__ == '__main__': guard. This reduces global pollution and prevents your code from running accidentally.
When you see something like:

if key in my_dict:
    my_dict[key] += value
else:
    my_dict[key] = default + value


Then you should probably use dict.get which can get the following code:

my_dict[key] = my_dict.get(key, default) + value


In this case you can add more sugar by using collections.defaultdict, as it will default missing keys to 0.

import collections

my_dict = collections.defaultdict(int)
my_dict[key] += 1

Don't use comprehensions with side effects, they're hard to understand and are better expressed as standard for loops. This is as the list you're making is absolutely pointless.
You can use dict.items() rather than your comprehension with dict.keys().


import re
import collections


def main():
    with open('mailbox.txt') as mbox:
        ndict = collections.defaultdict(int)
        for line in mbox:
            domain = re.findall('From [^ ].*@([^ ]*)', line)
            if domain:
                ndict[domain[0]] += 1

        for item in sorted(ndict.items(), key=lambda x: x[1], reverse=True):
            print(item)


if __name__ == '__main__':
    main()


You can replace the majority of your code with collections.Counter.

import re
import collections


def main():
    with open('mailbox.txt') as mbox:
        counts = collections.Counter(
            domain[0]
            for domain in re.findall('From [^ ].*@([^ ]*)', line)
            if domain
        )
        for item in counts.most_common():
            print(item)


if __name__ == '__main__':
    main()