code: def ValidateEmail(email, column_name=None, problems=None):
    
    if IsEmpty(email) or re.match(r'[^@]+@[^@]+\.[^@]+', email):
      return True
    else:
      if problems:
        problems.InvalidValue(column_name, email)
      return False
docs: checks the basic validity of email:
      - an empty email is considered valid and no error or warning is issued.
      - should start with any string not including @
      - then should match a single @
      - then matches any string not including @
      - then contains a single dot
      - then again matches any string after dot.