class GmailClient:
    """Authenticated Gmail client for one account (API or XOAUTH2 SMTP)."""

    def __init__(self, account_email: str, access_token: str, refresh_token: str,
                 client_config: dict, sending_method: str = "api", token_expiry=None):
        self.account_email = account_email
        self.client_config = client_config
        self.method = sending_method if sending_method in ("api", "smtp") else "api"
        creds = gcred.Credentials(
            token=access_token,
            refresh_token=refresh_token or None,
            token_uri=GOOGLE_TOKEN_URL,
            client_id=client_config["client_id"],
            client_secret=client_config["client_secret"],
            scopes=GMAIL_SCOPES,
        )
        self._creds = creds
        self._service = None
        self.token_expiry = token_expiry

    def _ensure_valid(self):
        request = google.auth.transport.requests.Request()
        if not self._creds.valid:
            self._creds.refresh(request)
        return self._creds.token

    def _api(self):
        if self._service is None: