input: Write a function to validate a list of emails. The function should return a boolean indicating whether a given email address is valid. If the given list is empty then return True.
  assert validate_mail_list(["example.user@gmail.com", "bad@example"])==False
  assert validate_mail_list(["valid.email@domain.com", "another.valid.email@sub.domain.com"])==True
  assert validate_mail_list([])==True
  assert validate_mail_list(["invalid_email"])==False
output: def validate_mail_list(emails):
      if not emails:
          return True
      import re
      pattern = r'^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$'
      for email in emails:
          if not re.match(pattern, email):
              return False
      return True