Return list of commercial email domains, which means:
    - domain is not public
    - domain is not university
    - it is not personal (more than 1 person using this domain)
    >>> "google.com" in commercial_domains()
    True
    >>> "microsoft.com" in commercial_domains()
    True
    >>> "isri.cs.cmu.edu" in commercial_domains()  # university department
    False
    >>> "jaraco.com" in commercial_domains()  # personal
    False

def commercial_domains():
    # type: () -> set
    """ Return list of commercial email domains, which means:
    - domain is not public
    - domain is not university
    - it is not personal (more than 1 person using this domain)
    >>> "google.com" in commercial_domains()
    True
    >>> "microsoft.com" in commercial_domains()
    True
    >>> "isri.cs.cmu.edu" in commercial_domains()  # university department
    False
    >>> "jaraco.com" in commercial_domains()  # personal
    False
    """
    dus = domain_user_stats()
    es = "test@" + pd.Series(dus.index, index=dus.index)
    return set(
        dus[~is_public_bulk(es) & ~is_university_bulk(es) & (dus > 1)].index)