from collections import Counter



def get_domain(email_address):
    """split on '@' and return the last piece"""
    return email_address.lower().split("@")[-1]


email_address="xxx@YYY.zzz"
x = email_address.lower().split("@")
print(x)
print(get_domain(email_address))
print()


c = Counter()
print(c)

# This is how **I** would do it...
with open('input.txt', 'r') as f:
    for line in f:
        if "@" in line:
            c[get_domain(line.strip())]+=1
            print(c)

#####################################################
# This is too Pythonic for my procedural mind...

#with open('input.txt', 'r') as f:
#    domain_counts = Counter(get_domain(line.strip())
#                            for line in f
#                            if "@" in line)
# print(domain_counts)