def longest_common_domain(email1, email2):
    a = email1.split('@')[1]
    b = email2.split('@')[1]
    # Compare strings at the C speed
    if a == b:
        return a
    min_len = min(len(a), len(b))
    i = -1
    while -i <= min_len and a[i] == b[i]:
        i -= 1
    return a[i:][1:]


test1 = 'user01@dev.connex.io', 'admin@connex.io'
test2 = 'testtest@gmail.com', 'foobar@mail.ru'
test3 = 'testtest@gmail.com', 'foobar@gmail.com'
test4 = 'testtest@4mail.com', 'foobar@5mail.com'

result = longest_common_domain(*test1)
print 'result:', result
assert result == 'connex.io'

result = longest_common_domain(*test2)
print 'result:', result
assert result == ''

result = longest_common_domain(*test3)
print 'result:', result
assert result == 'gmail.com'

result = longest_common_domain(*test4)
print 'result:', result
assert result == 'mail.com'