def METHOD_NAME(provider_name):
    """
    Return an authentication handler for the given provider.
    Params:
        provider_name (str): Name of the email provider ("custom", "gmail" or
            "outlook").
    Returns:
        An object that implements the AuthHandler interface.
    """
    if provider_name == "custom":
        from .generic import GenericAuthHandler
        return GenericAuthHandler()
    elif provider_name == "gmail":
        from .google import GoogleAuthHandler
        return GoogleAuthHandler()
    elif provider_name == "microsoft":
        from .microsoft import MicrosoftAuthHandler
        return MicrosoftAuthHandler()
    raise NotSupportedError(
        f'Nylas does not support the email provider "{provider_name}".'
    )