@staticmethod
def list_to_string(recipient):
    """
        Format the recipient for the MIMEMultipart() email type.
        If the recipient is a list, then it returns the list as a
        comma separated string.
        example: input=['you@here.com', 'her@there.com']
                 output='you@there.com, her@there.com'
        """
    if recipient:
        if isinstance(recipient, list):
            return ', '.join(recipient)
        return recipient
